190 



to generation is along a line of unicellular organisms, which are ever 

 such, and have been such for untold millions of years, until one or 

 other of them gets into the cul-de-sac of embryo -formation. It has 

 been indicated, that what we term the formation of an embryo is the 

 unfolding \) or evolution of one primary germ-cell, in order to furnish 

 a harbour of refuge for a certain set of germ -cells during a portion 

 of their life-cycle. 



Since writing in this sense almost two years ago, the facts ob- 

 served have only served to accentuate the importance of the conclu- 

 sions then arrived at. And in two directions in particular, apart from 

 others, such as the determination of sex and the existence of fourfold 

 gametes in the Metazoa, the observations have continued to throw light 

 upon interesting questions. 



Not very much experience with the work of enumerating germ- 

 .cells is needed to awaken suspicions, that the number is a constant 

 quantity, 2^, for the individuals of any particular species, and that it 

 varies in different species. The following out of these points leads, as 

 will be seen, to the enunciation of a numerical law of the germ-cells. 



In the second place, the tabulation of the germ -cells often goes 

 to show a disparity 1) in their arrangement on the two sides of the 

 body, and 2) in the total number of the normally placed, and, there- 

 fore, functional germ-cells of different embryos. The further consider- 

 ation of this latter point may be taken first of all. 



The theoretical aspect of the matter is as follows. Owing to the 

 circumstance, that in the skate, for example, the germ-cells are obliged 

 to migrate into the embryo, soon after the unfolding of this has been 

 initiated, — and of this immigrative process numerous preparations 

 from many embryos of 4 — 8 mm have now been obtained — it would 

 not be very surprising to find cases, where the embryo was entirely 

 sterile, or almost so, upon either one or both sides of the body. The 

 accident of circumstances furnished the means of testing this idea. It 

 is now more than twelve years, since the writer began to cultivate 

 and rear Elasmobranch embryos, more particularly those of Raja. In 

 the interval some thousands of eggs must have been opened. Ab- 

 normalities have all along been somewhat rare, and not until last 

 spring, in the course of a cultivation, undertaken on behalf of Prof. 

 DoHRN and myself, was anything very unusual noted. 



1) As an advocate o£ unfolding or evolution ray view of the de- 

 velopment is in accord with that of Weismann: the difference is simply, 

 that the evolution, attributed by him to the fertilised egg, is by me 

 assigned to one of the primary germ-cells arising from its cleavage. 



