The Organization of the Egg 191 



cells are separated, each one will develop into a half larva, 

 with only four rows of bands. Similarly each cell of the 

 four and even the eight cell stage may be made to develop 

 into a partial larva with two or even only one row of bands. 

 And further if a part be removed from the egg before divi- 

 sion, a defective larva is the result. 



The egg of the ascidian Cynthia has been shown by Conklin 

 to contain at least five different "organ-forming substances," 

 distinguishable by color and texture, which are symmetrically 

 placed with reference to the median plane of the embryo, 

 but differentially located antero-posteriorly. If one of the 

 first two cleavage cells (for example the right) is killed, the 

 other develops into the opposite (left) half of the body, which 

 contains all the normal parts, but of one-half the normal 

 size. But if in the four cell stage, when the second cleavage 

 has differentiated the anterior from the posterior ends of the 

 body, one or both of the anterior or posterior cells is killed, 

 the resulting larva lacks those parts which are present only 

 in the cells which have been destroyed. 



A similar result has been obtained by Wilson in the egg 

 of the mollusc Dentalium, in which three different substances 

 can be identified. Thus the yolk is here at first located at 

 one pole of the egg, and later in a single one of the cleavage 

 cells. If this "yolk lobe" be removed from the egg, when 

 it starts to divide, the resulting larva lacks certain parts 

 (foot, mantle, shell, etc.) normally formed from the yolk 

 cell. 



Centrifuging the egg of Cynthia with consequent dis- 

 arrangement of the organ-forming substances may so 

 disturb the development that the resulting larva may be 

 turned inside out, with entoderm on the outside and ectoderm 

 within. 



At the posterior end of one of the chrysomelid beetles 

 (Calligrapha) occurs a disk of granules, which seemingly 

 function as germ cell determinants, for if the end of the 

 egg containing this disk be pricked, and its component gran- 

 ules allowed to escape, or if the disk be destroyed with a hot 

 needle, and the egg is then allowed to develop, the resulting 

 embryo lacks germ cells. 



There are many experiments however which point to dif- 

 ferent conclusions. Thus in eggs of fresh water snails and 

 certain annelids, substances of different color and specific 

 gravity occur, but these may be displaced from their normal 

 positions by centrifuging, without in any way affecting the 

 development. This has led Lillie and others to the conclusion 

 that the so-called organ-forming substances are not in 

 reality such, but merely an accompaniment of a more pro- 

 found organization resident in the protoplasmic framework 



