Chap. VI.J GENERAL EMBRYOLOGY. 91 



planes intermediate between the first two. By them the eight 

 cells are divided into sixteen. Then follow two more horizontal 

 planes midway between the equator -and the poles. Thus 'we 

 get thirty-two cells. So the process continues, until by fresh 

 vertical and horizontal planes of division the ovum is divided 

 into a great number of cells. 



But meanwhile a cavity has formed in the midst of the ovum. 

 This makes its appearance at about the eight cell stage, the 

 eight cells not quite meeting in the centre of the ovum. The 

 central cavity so formed is thus surrounded by a single layer of 

 cells ; and it remains as a single layer throughout the process of 

 segmentation. So that the result of segmentation is the pro- 

 duction of a hollow vesicle (Fig. 32, i.) composed of a membrane 

 with a single layer of cells. Suchji vesicle is called a Uastosphere ; 

 the period during which this state of things continues is called 

 the Uastula stage ; and the cavity of the blastosphere is called 

 the segmentation cavity or Uastocode.^ 



Now suppose 1 that one side of the blastosphere, which is 

 represented in Fig. 32, ii., being composed of somewhat larger 

 cells, is pushed in; just as one might take a hollow squash india- . 

 rubber ball, and push in one side so as to form a hollow two- 

 layered cup. Suppose, moreover, that the mouth of the cup 

 closes in so as to become much smaller. Then we have the 

 state of things shown in Fig. 32, iii. This is the gastrula stage. 

 Thus we have constituted, by a process termed imagination, an 

 embryo with two layers of cells, an outer layer or epiblast (ep.\ 

 and an inner layer or hypoblast (%.). They are the two primi- 

 tive germinal layers. The cavity within the embryo is the primi-- 

 tive digestive body-cavity or archenteron (a. e,). The opening to 

 the exterior is the Uastopore (U.p.}. 



A transverse section through the middle of the embryo at the 

 gastrula stage shows a central cavity surrounded by a two- 

 layered body-wall. This state of things does not long continue. 

 Along a definite line on the surface of the embryo, marking off 

 a region henceforth to be known as the dorsal, the epiblast, or 



1 The advanced student will ascertain for himself how far these suppositions 

 are gratuitous. 



