REGENERATION IN EGG AND EMBRYO 239 



half-larva. He also inferred from certain incomplete embryos caught 

 in the sea, that these incomplete larvae could subsequently regener- 

 ate the missing parts. Driesch and Morgan ('95) studied the devel- 

 opment of the isolated blastomeres of another ctenophore, Bcroe ovata. 

 They found that the isolated one-half blastomere divides exactly as a 

 half of the whole egg (Fig. 66, A, B, C). It remains more or less a 

 half-structure, even after the ectoderm has grown over the whole sur- 

 face (Fig. 66, D). The invagination of ectoderm, to form the so-called 

 stomach, that takes place at the lower pole of the whole embryo, is 

 formed at one side of the lower pole in the half-embryo (Fig. 66, F, G). 

 It pushes into the endodermal yolk mass, and lies not in the middle, 

 but somewhat to one side. In the normal embryo there are formed 

 four endodermal sacs or pouches in the central yolk mass that become 

 connected with the inner end of the ectodermal stomach, around 

 which they lie symmetrically. In the half-embryo two sacs are 

 formed, and in addition a smaller third sac, which always lies on the 

 side of the stomach that is nearest the outer wall (Fig. 66, F, G). The 

 embryo is, therefore, somewhat more than half the normal embryo in 

 regard to the number of its endodermal sacs. 



There are present eight meridional rows of paddles in the normal 

 embryos of the ctenophore. They lie symmetrically on the sides, 

 converging towards an apical sense organ. In the one-half larva 

 there are always only four of these rows of paddles that are not 

 equally distributed over the surface, since on one side there is a wider 

 gap between two of the rows than elsewhere (Fig. 66, G}. The 

 sense plate also lies somewhat eccentrically, i.e. more towards the 

 side corresponding to that at which the other blastomere lay. 



If the one-fourth blastomeres are separated, each continues to seg- 

 ment as though still a part of the whole. A one-fourth embryo 

 develops that has an un symmetrical stomach, with- two endodermal 

 sacs. There are only two rows of paddles. The embryos are, there- 

 fore, in several respects one-fourth embryos, but the presence of two 

 endodermal sacs, instead of only one, shows that in this particular, 

 at least, the embryo is more than a fourth of the whole. 



The part of the work of Driesch and Morgan, that has a special 

 bearing on the interpretation of the one-half and one-fourth develop- 

 ment of the isolated blastomeres, is that in which some experiments 

 are described which consisted in cutting off portions of the unseg- 

 mented egg. If a fertilized but unsegmented egg is cut in two by 

 means of a small pair of scissors, the part that contains the nucleus 

 may segment, and give rise to an embryo. The division is generally 

 like that of a part, and in such cases an incomplete embryo develops. 

 The embryo may have fewer rows of swim-plates than has the normal 

 embryo, and fewer endodermal sacs, and the stomach may be in an 



