XYII PKOTOCHOEDATA 589 



spherical blastula. At this stage, if we compare the blastula to a 

 school globe, arid the animal pole to the North pole, and if we 

 imagine a traveller passing from the North pole to the South, he 

 would continually encounter larger cells ; whilst if he passed round 

 the equator from a point which we may compare to Africa, to a point 

 which we may compare to South America, he would also pass from 

 smaller to larger cells since the two anterior blastomeres of the 

 4 -cell stage, and all the cells descended from them, are smaller 

 than the two posterior blastomeres and their descendants. 



In the next stage the blastula flattens on one side and becomes 

 hemispherical (Fig. 429, A). On the flat side there are the largest 

 cells, which are now of a tall columnar shape. At one edge of the 

 flat surface there is an abrupt passage from cells of this character to 

 comparatively small cells, but at the other edge the tall columnar 

 cells pass gradually into the lower cells which form the hemispherical 

 wall of the blastula. The first edge of the flat surface we may term 

 x, the second y. 



The process of gastrulation begins within an inflection of the 

 flat surface near the edge named x (Fig. 429, B). The invagination 

 is therefore not a symmetrical invagination of the centre of the 

 lower surface, as in Balanoglossida and Echinodermata, but is such as 

 to give the impression of its being due to the push of an invisible 

 finger directed against the flat surface near the edge x. 



This asymmetry in the invagination was not noticed by 

 Kowalevsky and Hatschek, but was first noted by Lwoff, and is 

 made by him the foundation stone of his theory. According to him 

 the invagination occurs in two stages : in the first, the columnar 

 cells, which according to him alone represent the endoderm, are 

 invaginated. Following this stage, however, small cells are inflected 

 at the dorsal lip of the blastopore, and these Lwoff regards as 

 ectoderm. These small cells form eventually the roof of the archen- 

 teron, and from them notochord and mesoderm are developed, and these 

 structures therefore, on this theory, would be of ectodermal origin. 

 With this view Cerfontaine (1907) substantially agrees, except that 

 he maintains that, at a later stage in gastrulation, ectoderm is in- 

 vaginated round the ventral lip of the blastopore as well as round the 

 dorsal lip. 



Now, it will be observed that Lwoff s case stands or falls with the 

 presumption that, in the hemispherical stage of the blastula, ectoderm 

 and endoderm are already finally separated from one another. If 

 this presumption is ill-founded the whole procedure of splitting the 

 process of gastrulation into two stages is condemned. A careful 

 examination of the hemispherical blastula proves that, except at the 

 edge x, there is no such sharp delimitation of the columnar cells 

 which make up the supposed endoderm, from the supposed ectoderm, 

 as Lwoff postulates. The delimitation at x is seen to be due to the 

 presence there of a zone of rapidly divicfing cells, and it is plausible 

 to associate the invagination itself with the pressure exerted by the 



