142 EMBRYOLOGY 



cap upon the mass of macromeres, which it covers over (Fig. 

 65 M, Fig. 66 B and C). 



We may from now on consider this cap-like fundament of 

 micromeres, in accordance with its destiny, as ectoderm. It 

 spreads out more and more, especially by the progressive 

 growth of its marginal parts, so that it soon envelops not 

 only the upper portions, but also the lateral parts, of the 

 mass of macromeres (Fig. 66 G and 67 A). 



In this way the latter moves more and more into the in- 

 terior of the embryo, so that we here see a two-layer em- 

 bryonic form (gastrula) produced by means of circumcres- 

 cence or epiboly. Frequently the forward growth of the 

 margin of the ectoderm does not proceed uniformly at all 

 points, but a more active marginal growth is shown at points 

 corresponding to the four radii. Finally the macromeres 

 are seen to be covered by ectoderm on all sides except the 

 lower surface, where the ectoderm still presents a large 

 circular gap (Fig. 67 A), which we may designate as the 

 gastrula-mouth or blastopore. Up to this time the domi- 

 nant activity of the embryo consisted in the production of 

 ectodermal elements. The macromeres were involved in 

 this only in so far as they constantly budded off new ecto- 

 dermal elements from their upper surfaces. When the stage 

 last mentioned has been reached, this kind of increase of the 

 ectoderm ceases, and the macromeres from now on become 

 active in another direction. It is noteworthy, in contrast 

 with the considerable multiplication of the ectoderm cells, 

 that up to this time the eight macromeres have undergone 

 no increase in numbers. But now they begin to divide, so 

 that stages with twelve and then with sixteen macromeres 

 can be observed. Afterwards the division of the macromeres 

 becomes irregular. Meanwhile the basket-like arrangement 

 of the macromeres has vanished, and they now form a more 

 placentiform mass (Fig. 67 A). 



We have designated the micromeres as ectoderm ; we 

 could not, however, employ the term " entoderm " for the 

 macromeres, because, on the one hand, they still contained 

 parts which w^ere to be constricted off by budding and added 

 to the ectoderm, and because, on the other hand, as we know 



