THE VITAL IMPETUS 



129 



Unsegme 

 Ovum_ 



ated 



2- blasfomere 

 stage. 



f-blaslomene 

 Sta.a,e. 



3 



IOOO 



cells. 



ovaries all the future generations ; with other conse- 

 quences which the preformationists did not hesitate 

 to accept. (2) The other view was the later one 

 of epigenesis : the egg was truly homogeneous and 

 the embryo grew from it. Obviously the acceptance 

 of this hypothesis led to vitalism, and we find that it 

 was abandoned just as soon as the embryologists recog- 

 nised that physics provided a corpuscular theory of 

 matter, when a return was made to the preformation 

 views of earlier times ; views which lent themselves to 

 the construction of a 

 mechanistic hypothesis 

 of development. 



We may state very 

 briefly the main facts of 

 the development of a 

 typical animal ovum, 

 such as that of the sea- 

 urchin. 



The fertilised ovum 

 divides into two (2) , and 



then each of these blastomeres divides again in a plane 

 perpendicular to the first division plane (3). The third 

 division plane is at right angles to the first two, and it 

 cuts off a tier of smaller blastomeres from the tops of the 

 first four. There are now (4) two tiers of blastomeres, 

 a lower tier of large blastomeres and an upper tier 

 of smaller ones. This is the 8-cell stage. Next, 

 each of these blastomeres divides in two simultaneously 

 so that the embryo now consists of sixteen cells. After 

 this the divisions proceed with less regularity, but 

 after about ten divisions the embryo consists of about 

 1000 cells (2 10 ), and these are arranged to form a 

 hollow sphere consisting of a single layer of cells. The 

 latter are furnished with cilia, and the whole embryo, 



Fig. 12. 



