196 



The material of Elasmobranch embryos, not to speak of those of 

 other forms ^), at the writer's disposal is not yet exhausted. None the 

 less, it cannot lie within the scope of the research to give full and 

 complete accounts of the conditions in a number of different species, 

 such as Scyllium, Pristiurus, Acanthias, and Torpedo. Examples of 

 these, and representatives of other groups, have received greater or 

 less attention for reasons connected with either the distribution of 

 the germ-cells, or their number. 



It is now proposed to use the results of various observations to 

 answer the question "is there a numerical law of the germ -cells?" 

 As a further aid in this the observations of certain other embryolo- 

 gists will be cited. 



My enumerations of the germ-cells of Raja batis had not pro- 

 ceeded very far, before it was noted, that there must be two numbers, 

 around one or other of which the total grouped itself in every case. 

 These were 512, and the half of this, 256. For sufficient reasons ray 

 former publication contains no hint of this, although at the time of 

 writing, in May 1900, it had been clearly recognised. From the start 

 it was suspected, that these numbers related to the future female and 



1) In this connection a brief reference may be made to Nussbaum's 

 recent publication. (Zur Entwickeluug des Geschlechts beim Huhn, 

 Verhandl. Anat. Gesellsch. Bonn, 1901, p. 38 — 40.) Like the author I 

 have devoted some attention to the germ-cells of the chick; because it 

 was the object, upon which so many of the earlier observations were 

 made. As he states, it is not at all a favourable material, and with the 

 methods as yet employed I have not obtained such complete results as 

 had been anticipated. With the same method the germ -cells of the 

 chick do not stain like those of Raja, and hence are not so easily seen. 

 NussBAU.M would appear to have incubated at a higher temperature than 

 the writer, who took that given in Keibel's "Normen-Tafel", in order 

 to obtain phases there figured. For this reason I cannot confirm his 

 discovery of germ-cells within the embryo at the commencement of the 

 second day, which appears to me a very early period, and one, at which 

 in most chick-embryos one can hardly speak of a splanchnopleure. He 

 also states, that they increase by mitosis, whereas in all the thousands 

 of primary germ- cells studied by me — and during the whole of the 

 second and third days in the chick all the germ-cells must be primary 

 ones — I have seen but one instance of the division of a 

 primary germ-cell, prior to the epoch, when they begin to form 

 secondary germ-cells. 



The chief fact of his paper, that the germ - cells of the chick are 

 present, and not all in the normal position, long prior to the appearance 

 of the germinal nidus, I can fully confirm. And here also, as Nüssbaum 

 insists, they migrate. 



