THE TOPOGENESIS OF THE EMBRYO 95 



mesenchyme, later reinforced by a second generation of cells 

 which have become free from the endoderm (Fig. 32 e), ac- 

 cumulate in two groups, to the left and right of the gut. We 

 have described above (p. 83) how the skeleton develops within 

 these two cell masses (Fig. 32 f). The calcareous rods of the 

 skeleton show a rapid increase in length, and where their ends 

 come into contact with the ectoderm, the latter bulges out, 

 forming the so-called arms- In this way the blastula develops, 

 via the gastrula stage, into a pluteus larva which already 

 possesses the primordia of several organ systems characteristic 

 of the sea urchins (Fig. 10, 11, 15, and 29). This development 

 is achieved by a series of folding, invaginating and shifting 

 processes. 



In amphibians, gastrulation is somewhat more complicated. 

 Cleavage here results in a blastula with a multi-layered wall. 

 Its cavity (hlastocoel) is eccentric, lying nearer to the animal 

 pole of the egg. At this end, it is bordered by several layers of 

 small cells, whereas the floor of the cavity is formed by several 

 layers of large cells which are rich in yolk (Fig. 34a). 



The size of the cells increases gradually from the animal pole 

 towards the vegetative pole. The same statement applies to 

 their yolk content. In many amphibians, the original, bilateral 

 structure of the egg can be discerned in the blastula, because 

 the grey crescent, which was formed soon after fertilisation, is 

 still present. This marks the dorsal side of the egg. 



Here, again, a complicated system of cell movements begins 

 after the completion of cleavage, which shifts the cell material 

 of the prospective organs towards their appropriate places. 

 The best way to elucidate this complicated process is to consider 

 the surface of the blastula to be divided into three parts by 

 two imaginary lines. These parts are: (1) a marginal zone 

 which surrounds the egg in the vegetative half, and is more 

 or less parallel to the equator; it is wider at the dorsal side, 

 and here it extends into the animal half; (2) the animal fields 

 consisting of small, usually darkly pigmented cells; (3) the 

 vegetative field, consisting of large, unpigmented cells, con- 

 taining much yolk (Fig. 33). The boundaries of these areas 

 cannot be distinguished in the egg by their outward appear- 



