Mosaic Development in Ascidian Eggs. 163 



histological character of the cleavage cells nor their developmental 

 tendencies. 



h. Gastrulation. 



In the development of the right or left half of an egg the process 

 of gastrulation sometimes occurs in an unusual manner. The 

 most frequent modification of the normal process is that shown in 

 Figs. 27, 29, 30, where the endoderm cells are not infolded but 

 come to protrude above the level of the other cells, thus forming 

 exogastrulae. In later stages these endoderm cells must become 

 infolded for it is a rare thing to see exogastrulae or any indication 

 of an original evagination of endoderm cells in any of the cultures 

 of older embryos. By what process these exogastrulae right them- 

 selves I have not been able to observe, but I think it probable that 

 this like normal gastrulation is accomplished by overgrowth of 

 the ectoderm cells and change of shape of the endoderm cells. 



Sometimes when the endoderm cells are evaginated other por- 

 tions of the blastula wall invaginate. In this way false gastrulae 

 may arise in which the infolded cells are not endodermal but 

 ectodermal, as is clearly shown by their histological structure. 

 (Fig. 63.) 



While some embryos in the gastrula stage show such abnormali- 

 ties as those which have just been described in other cases the 

 gastrula is strictly a half one, as is shown in Fig. 31, and it seems 

 to me probable that exogastrulae or false gastrulae only arise when 

 the surviving half of the egg has been slightlv^ injured. These 

 half gastrulae contain just one-half of all the cells of the normal 

 gastrula and the position of the various cells and organ bases is 

 essentially like that which occurs in the right or left half of a 

 normal gastrula; the cells of the yellow crescent lie along one side 

 only of the blastopore groove; the neural plate and chorda cells 

 each form half of the arc which is normally present in the anterior 

 lip of the blastopore, while the closing of the open side of the 

 gastrula, which is turned toward the injured cell, is chiefly accom- 

 plished by the overgrowth of the ectoderm cells of the ventral 

 side. (Fig. 31.) 



Except, therefore, for this tendency of the cells along the injured 

 side to come together, these half gastrulae are strictly partial and 

 the gastrulation no less than the cleavage may be regarded as an 

 illustration of mosaic development. 



