320 Charles Zeleny. 



specimen. The experiment seems to indicate that the basis of the 

 apical organ is found in the four upper cells of the sixteen-cell 

 stage. In connection with this result Yatsu's observations on the 

 unsegmented egg of C. lacteus are interesting. He localizes the 

 basis of the apical organ in a broad band just above the equator 

 of the egg. 



X. Blastula stage. Successful operations were made on three 

 blastulae. 



The first one was divided by a horizontal cut into an upper part 

 ( = 2/3 of blastula) and a lower part ( = 1/3 of blastula). The 

 orientation was made certain by the presence of the polar bodies. 

 The upper part broke up into two portions, each of which at 

 twenty-four hours had developed into an embryo with an apical 

 cilium. At the same time the embryo from the lower one-third 

 of the blastula was ciliated but had no apical organ. At forty- 

 seven and a half hours the embryo from the lower one-third and 

 one of the upper ones were dead. The other upper embryo has 

 two apical plates, one a well developed and the other a small one, 

 an invaginated ectodermal sac, a large and well developed en- 

 teron, a blastocoele with free cells in its cavity, and no lappets. 

 In fact, except for the absence of the lappets and the presence of 

 two apical organs, it has all the characters of a typical whole 

 larva (Fig. 19C; only one of the apical plates is shown). How- 

 ever, at twenty-four hours, as stated above, there is a distinct dif- 

 ference between the upper embryos and the lower one because of 

 the presence of the apical organ in the former and its absence in 

 the latter. 



A second blastula was cut into equal upper and lower parts by 

 a horizontal cut, but the two were not kept separate. One of the 

 halves died. The other developed all the organs of the normal 

 pilidium, except the lappets. There is a large blastocoele, two 

 apical organs, one in the normal position and one asymmetri- 

 cally placed and not shown in the figure, and a large long enteron 

 straighter than in the normal larva (Fig. 19B). 



A third blastul-a was cut into two unequal parts equal respec- 

 tively to two-thirds and one-third of the blastula, by a cut of un- 

 known direction. One portion, the larger one judging by the 



