52 



Pristiurus is not in some respects a particularly favourable animal 

 for the investigation of germ-cells. Relatively to corresponding ones 

 of Raja batis its embryos are very small, and, although its germ-cells ^) 

 are nearly as large as those of the latter, although they stain in the 

 same way and stand out with the like clearness in properly stained 

 specimens, the crowding - together of them, which happens at certain 

 periods of the development, serves to increase the difficulties of an 

 enumeration. In this form, and, indeed, in all the others examined 

 including Raja batis, no pretence is made of absolute arithmetical ac- 

 curacy in the results obtained. They may be regarded as careful 

 estimates, rather than infallible enumerations. The task of counting 

 germ-cells is not among the most agreeable, or even exciting, of micro- 

 scopical studies, and from ample experience of it the writer is con- 

 vinced, that nearly always the total found is beyond the mark, rather 

 than below it. 



From the circumstance, moreover, that germ-cells may degenerate 

 at any period of the life-history, even before the actual unfolding of 

 an embryo, and that one can never be absolutely certain, that every 

 single one of the germ-cells is within the embryonic boundaries (i. e., 

 that some of them may not be in the blastoderm, or even in the yolk), 

 it will be evident, that their enumeration is not comparable to, for 

 example, one of the marbles in a schoolboy's pockets. 



In his memoir Rabl ('96) treats in detail of 22 embryos, seven 

 of which are larger than the oldest one to be described here. For 

 the portion of the development lying between the closure of the me- 

 dullary folds and embryos of 16 mm against Rabl's 15 embryos there 

 may be set the 16 to be treated of here. The embryos, dealt with 

 in the present writing, are simply the majority of the first sending 

 (21 embryos) received from Naples. Of these 21 the ones not used 

 here are a disc of the cleavage-period and 3 embryos not yet examined. 



It has not been found possible as yet to count the germ-cells of 

 embryos of Pristiurus of less than 5 mm. Prior to this period their 

 distribution is a very wide one, as will presently appear, and those in 

 or near the normal position, the site of the future germinal nidus, are 

 often so closely aggregated, that an estimate of their actual number, 

 without making reconstructions, is very difficult. In embryos of 5 mm 

 and older up to such of 16 mm the enumeration is more successful. 

 But here, as in Raja and other forms, this only becomes possible in 

 sections of a fair degree of thickness (i. e., 130 sections or fewer in 



1) 0.018-0.02 mm. 



