170 



itta, a pelagic free-swimming form. la vertebrata they do not become dis- 

 tinct from the cells of the body until the embryo is completely formed." 

 It will be seen that in some vertebrates (Cf/matogasfer) a similar segregation, 

 of " germ plasm " takes place quite early. In brief, the sex-cells of Cumato- 

 '/aster first become normally conspicuous in the mesoblast where the germ 

 layers are fused before any protovertebra- are tormed. They can be seen 

 in earlier stages, but they do not stand out so prominently from the other 

 cells. In exceptional cases, the sex-cells can be traced back to probably 

 the fifth segmentation. 



The sex-cells can first be distinguished from the surrounding cells about 

 the time the blastopore closes. The earliest ones distinguishable, exclu- 

 sive of abnormal cases, are from an ovary in the eggs of which the blasto- 

 pore is not yet closed, or is just closed and in which the mesoderm is not 

 yet split off from the entoderm. Only two cells which can with certainty 

 be said to be sex-cells are seen in one of these eggs. They differ from 

 the surrounding cells in having well-defined, rounded outlines, and in 

 the distribution of the chromatin in the nucleus. The chromatin of Ihe 

 surrounding cells is collected in one, or, if the cells are undergoing divis- 

 ion, in two or three masses. The chromatin of the sex-cells is uni- 

 formly distributed in email granules. 



In the eggs of another ovary, in which thickenings are formed for some 

 distance, and the mesoblast is separated from the entoderm by a well- 

 marked line, the sex-cells stand out from the surrounding cells with great 

 prominence. This is not due to any marked change in the sex-cells them- 

 selves, but rather to the fact that the surrounding cells have undergone 

 further division and are crowded so that the boundaries are not defined , 

 while those of the rounded sex-cells are well marked. 



The largest and most conspicuous cell of this stage lies in the mesoblast 

 just beside the chorda. It measures 18x23 m, and has a nucleus measuring 

 about 6 }n. On comparing this with segmenting eggs, it is found that 

 it agrees in size with some of the cells of an egg undergoing the ninth seg- 

 mentation and in all probability it is a cell remaining unchanged from that 

 stage. It contains yolk particles. Most of the sex-cells are collected in a 

 limited region at this stage in the thickened portion of the embryo, where 

 the three germ layers fuse. This would lend force to the supposition that 

 they are derived from two cells at most— one dextral and one sinistral. 

 There are a few scattered cells in other parts of the embryo which cannot 

 be so derived unless they early migrate from their original position. 



