THE TOPOGENESIS OF THE EMBRYO 93 



to their appropriate places in the structural plan. These move- 

 ments may take many different forms, such as folding, in- 

 vagination, or shifting, either of cell groups, or of isolated cells. 



In the eggs of most animals, a period sets in, directly after 

 the completion of cleavage, in which these movements dominate 

 the whole picture of development. So far, the egg has been 

 rather inert, cleavage being its only manifestation of activity. 

 Now, however, it suddenly seems to wake up, and to begin its 

 real task, the formation of an embryo. The whole set of topo- 

 genetic phenomena immediately following cleavage is called 

 gastrulation. Cleavage had already produced a vesicular germ, 

 the blastula. During gastrulation this enters upon a phase in 

 which some of the main components of the future embryo can 

 already be recognized. We shall now discuss the process of 

 gastrulation in two animal groups which have often been used 

 for experiments in developmental physiology, viz. in sea urchins 

 and amphibians. 



The sea urchin germ at the end of cleavage is a vesicle with 

 a wall consisting of a single layer of cells (Fig. 32a). The cells 

 of the vegetative side are slightly higher than those of the 

 animal pole. The latter carry a long tuft of stiff cilia, whereas 

 the other cells have short, motile cilia. The beginning of gastrul- 

 ation is marked by an indentation at the vegetative pole of the 

 blastula. At first, this is shallow, but it penetrates gradually into 

 the interior, thereby forming the archenteron (Fig. 32 d, e). 

 The opening which connects the archenteron with the exterior 

 is called the blastopore. The cells which form the wall of the 

 archenteron are called the endoderm; to the cells remaining at 

 the surface of the germ the term ectoderm is applied. Further, 

 a small number of cells, originally occupying the vegetative pole 

 of the blastula, now disengage themselves from the epithelium, 

 and move into the space between ectoderm and endoderm. They 

 constitute the so-called primary mesenchyme (Fig. 32b, c). We 

 have already seen (p. 78) that the formation of the primary 

 mesenchyme may take place at different stages in development. 

 If this occurs at an early stage, the cells become free from 

 the blastula wall even before the invagination of the archen- 

 teron; if at a later stage, they are first carried along by the 



