618 HAROLD HEATH, 



Occasionally the first cells to invaginate are the fourth quartette 

 cells in quadrants Ä, B and C which in some cases disappear wholly 

 from view before any other cell exhibits a similar movement. In 

 other rare cases where the cells are of normal size and arrangement 

 the same thing may be said of the raesoblast, while in cases where 

 D or 2d^-^ are larger than usual the mesoblast usually passes into the 

 embryo at an early stage. But the normal invagination commences 

 shortly after the division of the third quartette stomatoblasts. Almost 

 from the moment of its formation the mesoblast has been slowly 

 moving inward but it is almost imperceptable and therein differs from 

 the general normal invagination movement which first affects all the three 

 macromeres Ä, B and G. These when compared with the mesoblast 

 glide with considerable rapidity past the fourth quartette cells, with 

 which they are in contact, and push into the segmentation cavity. 

 In the first stages of this movement the fourth quartette is not 

 affected but as the macromeres advance the former become gradually 

 depressed and slowly pass with them into the embryo. Shortly after 

 the movements initiated by the macromeres commence, macromere D 

 commences to move in the wake of the other macromeres, especially 

 if it be of considerable size; if it be small it may remain for some 

 time attached in its original position to the mesoblast. As the macro- 

 meres disappear from view it is seen that they gradually fill the 

 segmentation cavity, and what is now important they press in 

 above the mesoblast as in Fig. 37. Since their movement is 

 more rapid than the mesoblast there must result a tendency to check 

 the inward movement of these latter cells. At all events that is what 

 happens. From this time forward the movements of the mesoblastic 

 products become slower and slower and ultimately cease altogether^ 

 and therefore the invagination in the anterior quadrants A and B is 

 much more rapid than that in G and D. Considering A and B first 

 we find after the macromeres in their inward movement have been 

 followed a short distance by the fourth quartette cells, that a similar 

 movement occurs in the third quartette stomatoblasts. These are two 

 in number in each quadrant and when viewed from the vegetative 

 pole are at higher level than the fourth quartette cells and are also 

 above the narrow spaces that separate these latter cells from each 

 other. These spaces as the cells pass inward gradually enlarge and 

 into them the stomatoblasts finally work their way. Large as these 

 products are in Fig. 35 this process would probably be accomplished 

 with difficulty but at this point each of the two cells in each quadrant 



