86 J. Thos. Patterson. 



simply speak of the thinning-out process, by which I mean the 

 (iifferentiation of the cleavage cells into a single layered epithelium 

 above the enlarged segmentation cavity (subgerminal cavity). 



In this connection I must speak of the probable reason why the 

 thinning-out process affects the posterior region of the blastoderm 

 first. I have come to regard this as signifying that the posterior 

 region is farther advanced in its differentiation than other parts. 

 This interpretation is in harmony with the general law for the early 

 development of the embryo, namely, that differentiation progresses 

 from the head end backward. As we shall see later, it is in the 

 posterior central part of the blastoderm that the head of the embryo 

 will arise. 



h. Gastrulation Stages. 

 (1) Invagination. 



If the thinning-out were completed before the invagination began, 

 the interpretation of the steps of gastrulation would be greatly 

 facilitated. But such is not the case, for immediately following a 

 stage such as shown in Fig. 30, the initial step in gastrulation occurs. 

 This consists in the rolling under of the free posterior margin of 

 the blastoderm. The reconstruction of a blastoderm in which the 

 involution has just taken place is shown in Fig. VIII, and a surface 

 view of a corresponding stage is seen in Fig. V, G. In this egg 

 (Fig. VIII) the zone of junction is not essentially different from 

 that seen in Fig.' VII, except at the anterior inner margin, Avhere 

 a portion of it has given rise to a partial germ-wall (gw). The 

 numbers scattered over the figure indicate the relative depths of 

 the various regions. Thus in the central area, the blastoderm is 

 thinned-out to one or two cells, while the marginal parts are much 

 thicker, varying from two to four cells. In the extreme posterior 

 is shown the region covered by the invaginated entoderm (E). 



The posterior portion of an oblique section passing through the 

 region of invagination is represented in Figs. 32 and 15. At the 

 extreme posterior is a cavity (Fig. 15, c) which is bounded above 

 by the vitelline membrane and below by the yolk, or ventral lip of 

 the blastopore. In reality the cavity is but a portion of the blasto- 



