171 



There are, on an average, thirteen sex-cells in an egg of this stage. The 

 largest number noticed is seventeen, the smallest nine. 



In a larva just hatched, the longest diameter of which, measuring in a 

 straight line, is 0.45 mm., there are ten sex-cells. In this embryo about 

 nine protovertebra? have been formed. Most of the sex-cells are large, the 

 largest having a diameter of 23 rii, with a nucleus of 8 m. The smallest cell 

 measures but 11 ;/* in diameter. The distribntion of these cells has become 

 markedly changed from the conditions obtaining in the two-proto vertebra' 

 stage. Two of the cells, in the embryos examined, are found in the 

 cephalic region, one on either side a short distance posterior to the or- 

 igin of the chorda. The remainder are distributed as follows: one below 

 the seventh sinistral protovertebra ; three in the left side of the tail, i. e. 

 in the region in which protovertebrae have not yet appeared ; and three in 

 the right side of the tail. 



The cells in this stage stain deeper and much more uniformly than the 

 surrounding cells with Grenacher's hamatoxylin. They greatly resemble 

 the very early conditions of these cells, and the number would seem to in- 

 dicate that there has been no segmentation since the two-proto vertebra- 

 stage. In other larva^ of the same stage there are ten, eight, five, and nine 

 cells, respectively. 



In larvpe 2.5 mm. long there are fourteen to sixteen cells and the num- 

 ber cannot have been increased much since their earliest condition, even if 

 we assume that two or more have been lodged in the gill region, and two 

 in the anterior part of the body. The majority of the cells in this larva 

 are confined to a region only 0.20 mm. long; and if we consider the doubt- 

 ful cells in the anterior region, the total length over which these cells are 

 distributed is about 0.50 mm. from the anus forward. The sex-cells in this 

 stage measure 9-13 >//. Balfour's admirable account of these "primitive 

 ova" (Elasmobranch Fishes, pp. 130-13(5) might almost be used bodily to 

 describe the same structures in Cymalogadcr and Ahrona 2.5 mm. long. 

 He observed that the younger ones contain many yolk spherules, and sug- 

 gests that the cells themselves may have migrated to their position from a 

 peripheral portion of the blastoderm, since " they are the only mesoblast 

 cells filled at this period with yolk spherules." He was at a loss as to how 

 they arose, and thought he could detect cells intermediate in size between 

 them and the neighboring cells. As has been seen, the yolk particles sim- 

 ply remain unchanged from the original condition when the sex-cells are 

 segregated. 



