DEVELOPMENT OF EGG FRAGMENTS IN CEREBRATULUS. I 3 1 



In three cases I got two larvae from one fertilized egg. There 

 are three possibilities to explain this : first, when the operation is 

 done before the fusion of the egg- and sperm-nuclei ; second, 

 after the first cleavage mitosis came to the telophase ; third, when 

 the egg is doubly fertilized and the segmentation nucleus is cut 

 apart from the sperm-nucleus. Since my operations were done 

 not so late as the second case, the result may be due either to the 

 first or the third cause. Whatever the cause may be, the com- 

 parison of the resulting larvae is very interesting. ^.A and ^.A' 

 (Nos. 14 and 14') show a most instructive pair of pilidia. The 

 larger of the two is almost normal, except that the gut is very 

 defective, while the smaller one has a comparatively large gut. 

 Professor Wilson found a similar pair of larvae by cutting the 

 blastulas (cf. his Fig. 1 1, A and B}. Another pair (^B and ^B' , 

 Nos. 15 and 15') are also important; both of them are normal, 

 but one ciliated lobe is wanting in each. In still another pair, 

 4.C and 46"' (Nos. 23 and 23'), both are devoid of the apical 

 organ and have a very defective gut. Either of them is barely 

 more than the lappet. 



Now let us see how the basis of the ciliated lobes and gut are 

 disposed in the egg of this stage. Most of the defective larvae 

 have abnormalities in the ciliated lobes in some way or other. 

 We can distinguish two kinds of defect ; in one only one ciliated 

 lobe has been developed, the other being entirely suppressed, so 

 that the mouth can be seen from the side (^.A' , 4.8' , ^C and 

 4ZT). The other kind of defect is shown in ^.F. In this case 

 both the ciliated lobes are present, but they are very short and 

 almost straight. This difference may be ascribed to the fact that 

 the basis of the ciliated lobes is more or less bilaterally situated 

 in the vegetative hemisphere near the equator. As for the basis 

 of the gut I know very little, but it is certain that it lies near the 

 vegative pole (cf. ^A and 



SERIES D. 



Development of Fragments obtained before the Completion of the 



first Cleavage. 



The eggs were cut between the period of the appearance of the 

 first cleavage furrow and the completion of the division. This 



