222 ZOOLOGY. 



increases to sixty-four, one hundred and twenty-eight, and 

 two hundred and fifty-six. The resulting hollow sphere is 

 called a blastosphere, and the central cavity the segmenta- 

 tion cavity. The blastosphere stage is taken as the end of 

 segmentation pure and simple. 



Technically, the segmentation of the ovum of Amphioxus 

 is said to be holoblastic and nearly equal. The former term 

 means that the whole substance of the ovum divides up into 

 cells (in the fowl's egg, as we shall see, it does not). The 

 latter means that up to the end of segmentation all the 

 cells produced are nearly equal in size. Really, those in 

 the vegetative hemisphere are a little larger, but compared 



ABO 



Fig. 112. SECTIONS OF EMBRYO AMPHIOXUS. 



A, Blastosphere ; B, early gastrula; C, later gastrula, with ciliated 



epiblast. 



with the striking inequality of the segmentation in some 

 types this is insignificant. 



8. The Gastrula. We next notice (fig. 112, A), that the 

 vegetative end of the sphere is decidedly flattened. This 

 flattened area becomes concave (fig. 112, B), and finally seems 

 to bo tucked-in until it touches the animal hemisphere, and 

 the segmentation cavity is obliterated. As cell-division still 

 goes on, the embryo again comes to a nearly spherical shape 

 (fig. 112, c), but now the sphere consists of two layers of cells, 

 and the central cavity has an opening. In these stages the 

 embryo is called a gastrula : this stage is one of the greatest 

 interest, as such a great variety of animals go through it : 

 in fact it is characteristic of the development of the simpler 

 members of most of the great animal groups. Certain new 



