An Introduction to the Invertebrates, Second Edition - tiera.ru

248 DEVELOPMENTpolarity and the interactions of cells that give them positionalinformation.Growth: at each stage in the laying down of the basic pattern ofthe organism, there can be some growth. Cells increase in sizeand multiply, and extracellular materials (including fluidsand hard skeletons) are produced.All these processes achieve an ordered increase in biologicalcomplexity as the three-dimensional cell architecture of the egg isdivided into lineages of developing cells.19.2 What makes different animals developdifferently?Different invertebrate phyla often have characteristic developmentalmechanisms, as is explained below (and see Figures 19.1 to 19.4)but differences are not solely related to phylogeny. Different environmentsdemand major differences in strategies for reproductionand development, as is shown by comparison of marine, freshwaterand terrestrial animals (see Box 7.1 and Chapter 13). One strategicdifference is the quantity of yolk that the eggs contain. Largequantities of yolk feed the embryo for longer, allowing longerprotection within the egg with later hatching, as is often necessaryin fresh water and on land, but yolk is expensive to make and isheavy and inert, hampering cell division and cell movement. If thereis very little yolk, many more eggs can be produced, the embryo candivide faster and gastrulate unimpeded, but then it needs food.Most invertebrate eggs with little yolk hatch very early into a larvalform that can find its own food; a few terrestrial invertebrates areviviparous, the young being retained and fed by the mother withinher body: this occurs in some insects (for example tsetse flies) andin some species of Peripatus, where the young are fed on internallysecreted ‘milk’. Many invertebrates in various habitats retain andprotect developing eggs (‘ovoviviparity’), often in a brood pouch.Within a species also, environmental factors can modify development:for example, the polymorphism between females withina species of social insects may be environmentally determined, andtemperature changes may determine the rate and even sometimesthe kind of development, as in those vertebrates where sex isdetermined by ambient temperature.19.3 What is the pattern of cleavage ininvertebrates?Cleavage is the division of the fertilised egg to form a mass of cells,called ‘blastomeres’. All the products of cell division in the embryo