482 EIGENMANN. [Vol. V. 



at a very early period, although not quite at the beginning of 

 segmentation, i.e. when the egg has divided into not more than 

 thirty segments. Here also the cells which are separated early 

 form the reproductive organs of the animal. The separation of 

 the reproductive cells from those of the body takes place at a 

 still later period, viz. at the close of segmentation, in Sagitta, a 

 pelagic free-swimming form. In vertebrata they do not become 

 distinct from the cells of the body until the embryo is com- 

 pletely formed." It will be seen that in some vertebrates 

 (Micrometrus) a similar segregation of "germ plasm" takes 

 place quite early. In brief, the sex-cells of Micrometrus first 

 become normally conspicuous in the mesoblast of the head, 

 where the germ layers are fused before any protovertebrae are 

 formed. They can be seen in earlier stages, but they do not 

 stand out so prominently from the other cells. In exceptional 

 cases, as will be seen later, the sex-cells can be traced back to 

 probably the fifth segmentation. 



The cells can best be demonstrated by hardening twenty-four 

 hours in the strong mixture of osmic, chromic, acetic acid, fol- 

 lowed by Delafield's or Grenacher's hematoxylin. 



In order to elucidate the figures accompanying this paper, as 

 well as some parts of the text, it is necessary for me to give a 

 brief account of the early stages of the fish, since it differs so 

 essentially in general development and in every detail from that 

 usually obtaining in teleosts. 



The whole early development is an abbreviated one. To this 

 fact, and to the small yolk, are to be attributed most of the pecul- 

 iarities of its early stages. During the ninth segmentation the 

 outermost layer of cells begins to spread over the yolk. During 

 segmentation a mass of protoplasm fills a pit in the entodermic 

 pole of the egg. Near the end of the tenth segmentation the 

 horny layer forms a thin layer over the whole of the yolk exclu- 

 sive of this entodermic mass of protoplasm ; the deeper layers 

 now begin to spread over the yolk. During the thirteenth seg- 

 mentation the blastopore closes. At this time the embryo is 

 composed of two layers, a one-cell deep primitive entoderm of 

 small cells covering the yolk (in the embryonic axis this layer 

 is several cells deep) and a two to three cell deep ectoderm. 

 The growth is now so rapid that at the time Kupffer's vesicle 

 appears the larva is free from its zona radiata. The periblast 



