Picture - Cosmic Polymath

402 DESIGN IN NATURE
left to chance or even to the individual effort of any particular organism either during its developmental or adult
condition. In the case of the frog the embryo begins its career as a fish does. It has a ventral bag of pabulum
(vitellus), no alimentary canal or mouth, no tail, and no gills (branchiae). As development proceeds the bag of
pabulum disappears and a mouth and alimentary canal are formed, a swimming tail, and gills or water -breathing
outside lungs make their appearance. Further on, the tail shrinks and falls off, and four rudimentary hmbs are
provided instead. The gills or outside lungs adapted for water-breathing also give place to inside or true lungs
adapted for air-breathing. These changes, be it remarked, are not due either to the disuse or the inadequacy of the
tail and gills, but only to the exigencies of design and final purpose. They are means to ends, and the means are
employed to secure the ends in anticipation, and according to pre-arrangement. In the menobranchus (a fish-like
animal) the gills are persistent, and so are the limbs (Plate xciii., Fig. 4, page 40.3).
PLATE XCIII
Plate xciii. illustrates development and peculiarities in the fish, frog, menobranchus, and other animals.
Fifi. 1.—Egg of fish.
A. Egg of fish, showing formation of umbilical vesicle.
B. Young fish with umbilical vesicle.
C. Human embryo, with umbilical vesicle (about the fifth week).
Fig. 2.—Shows the development of the frog.
A. Diagram of frog's egg, in an early stage of development, as seen in longitudinal section, o, Thickened portion of external
blastodermic layer, forming body of fo?tus ; h, anterior extremity of fretus ; c, posterior extremity ; d, internal layer of blastodermic
membrane ; e, cavity of vitellus.
B. Egg of frog, in process of development ; tail appearing.
C.
D.
Egg of frog, further advanced ; eye appearing, tail increasing in size.
Tadpole fully developed ; eye more advanced, and mouth and alimentary canal formed.
Fig. .3.—Shows tadpole developing limbs and adult frog. In the first figure the tail is diminished in volume, and rudimentary
limbs are appearing. The gills or branohife are also in a transition stage ; in other words they are changing from outside waterbreathing
lungs to inside air-breathing ones. In the second figure the frog is fully developed. It has lost its tail and is provided
with four powerful limbs and true air-breathing lungs.
Pig. 4.—Head and gills of Menobranchu.t. In this curious creature the outside branchiae or water-breathing lungs and the
limbs are persistent.
Fig. 5.— Gills of a, ganoid fish (C'eratodus). Affords a good example of the outside or water-breathing lung. ((, Arcus aortse ;
/), gills containing rows of capillary blood-vessels ; c, pseudo-branchia ; d, d', two series of gill-rakers belonging to the pseudobranchia
; e, ceratohyal ; /, glossohyal (after Gunther).
Fig. 6.—Diagram of the larva of a calcareous sponge in its gastrula phase. The larva at this stage consists of a simple pouch
or stomach with a mouth (a). The walls of the stomach are formed of two layers of cells, an internal and an external ; the external
cells being elongated and furnished with filaments (cilia), which serve as organs of locomotion. These two layers of cells have been
likened to the internal and external layers of the blastodermic membranes in the higher animals (after Haeckel).
Fig. 7.—Ascidian.
A. Early tailed larva.
B. Tailed larva, seen on the right side (altered from Seeliger).
0. Transverse section through the tail of an ascidian larva (after Seeliger).
A. o. Brain ; h, neural tube ; c, notochord ; d, mesenteron.
B. a, Brain, with eye and ear ; h, nerve cord ; c, notochord ; d, atriopore ; e, intestine ; /, remains of caudal intestine ; g, neuropore
; h, papilla ; i, laryngeal clefts ; k, endostyle in wall of pharynx ; j, heart.
C. a, Median fin ; /), nerve tube ; c, notochord ; d, muscle ; e, caudal hypoblast. The parts here enumerated foreshadow
advancing types ; a vertebral column, nervous system, alimentary canal, vascular system, and sense organs being all indicated.
The ascidian is to be regarded as intermediate between the molluscoid invertebrates and the vertebrata.
Fig. 8.-0, h, r, d, Polar capsules of My:a)bohis ellipsoiden (after Balbiani). Illustrates spiral, vibratile swimming arrangements
in rudimentary structures.
Fig. 9.—Shows a series of embryos exhibiting similarity of structure and development in their primary and secondary stages.
the lower row shows the partial disappearance of the gill arches and
The upper row shows the gill arches and the absence of limbs ;
the budding of the limbs.
A. Gill arches and rudimentary limbs in tortoise.
B. Gill arches and rudimentary limbs in fowl.
0. Gill arches and rudimentary limbs in echidna.
D.
E.
Gill arches and rudimentary limbs in koala or Australian bear.
Gill arches and rudimentary limbs in rabbit.
F. Gill arches and rudimentary limbs in pig.
G. Gill arches and rudimentary limbs in deer.
H. Gill arches and rudimentary limbs in cat.
1.
J.
Gill arches and rudimentary limbs in monkey.
Gill arches and rudimentary limbs in man (after Haeckel).
Note.—-T\ie figures A to J of Fig. 9, representing stages of development in various animals, and consisting as they do of mere
repetitions of parts, indicate a general plan or type. They afl:ord no proof of evolution, or the production of the higher animal
forms from the lower ones. The upper and lower rows of figures, taken separately, show repetition as apart from advance.