Mosaic Development in Ascidian Eggs. 175 



endoderm cells and these from the deep gray material of the egg. 

 In spite therefore of the regulation which is apparent in the closing 

 of the open side of the embryo, and in the formation of a whole 

 notochord and of an imperfect sense vesicle, the various ooplas- 

 mic substances of the unsegmented egg and of the different 

 blastomeres are not totipotent but each shows in these experi- 

 ments, as well as in normal development, that it is differentiated 

 to give rise to one, and only one, particular kind of tissue. 



3. Anterior Half Embryos {Figs. ^7-52). 



The anterior and posterior half embryos show even more 

 clearly than do the lateral ones the mosaic character of the develop- 

 ment of these eggs. When the posterior half of an egg is killed 

 in the 4-cell or 8-cell stage the anterior half continues to develop 

 as if the posterior half were still living. The cleavage is in all 

 respects like that of the anterior half of a normal egg; the gastrula- 

 tion is essentially the same, but the later development is modified 

 in many important particulars. 



Figs. 47 and 48 are ventral and dorsal views, respectively, of one 

 and the same living embryo of the 76-cell stage, in which the 

 posterior dorsal cells, B^^ containing the yellow crescent, were 

 killed in the 8-cell stage. None of the cells of the ventral hemi- 

 sphere were injured and consequently the cleavage of these cells 

 is quite normal; thirty-two ectoderm cells are present, all of which 

 have entirely normal positions, shapes and sizes, {cf. Figs. 5 and 

 47.) The anterior half of the dorsal hemisphere is also entirely 

 normal {cf. Figs. 6 and 48); eight chorda cells are shown forming 

 an arc which bounds anteriorly the six endoderm cells and which 

 is flanked on each side by the anterior mesenchyme cell, A^'^. 

 The number, size and position of each and all of these cells is the 

 exact counterpart of what is found in the normal embryo, and, 

 although the outlines of the neural plate cells were so indistinct 

 in the living specimen from which this figure was made that I 

 could not draw them, there is every reason to suppose that these 

 cells like all the others in this embryo conform to the normal 

 type. 



In the posterior half of the dorsal hemisphere all the parts 

 which would have developed from the cells B^^ and B^^ are 

 entirely lacking; there are neither mesenchyme, caudal endoderm. 



