732 JOHN BEARD, 



difficult, or even impossible, to demonstrate the truth of this, but the 

 analogy of other cases points to its correctness. 



It was the recognition of the probability of the incompleteness of 

 BovERi's diagram ^) of oogenesis in this lespect, which caused the 

 delay in the publication of the diagram of the life-cycle of the skate. 

 The writer is now convinced, though he cannot at present demonstrate 

 it in the skate, that the actual differentiation of sex, its determination, 

 goes back to the final divisions of the oogonia, in all probability to 

 the very last one itself, before these become oocytes. This must be 

 so, because the germ-cells are unicellular organisms, and because their 

 life-cycles are those of such. For this reason the differentiation of 

 direct forerunners of unlike gametes must happen as it would do in 

 the Protozoa themselves. 



As additional evidence in the like direction the well known eggs, 

 of two kinds and sexually differentiated, of Hydatina senta, Phylloxera, 

 and DinopMlus apatris == gyrociliatus may be cited (Figs. 6 and 7). 

 The small eggs here are destined to produce males, the large ones 

 females. As the size of the egg will naturally be attained during the 

 oogenesis, it would seem to follow, that here the destination of the 

 oogonium must be determined prior to the final phenomena of the re- 

 duction and of the ripening, for these latter would not appear to 

 possess any influence on the size of the egg itself. In other words, 

 the size of the egg and its prospective destination, male or female, must 

 be "arranged" prior to the ripening and reduction, i. e., prior to 

 the two final mitoses. 



This epoch — the final division of the oogonia — is the latest 

 one, at which the determination of sex can conceivably happen. It 

 is in all probability the usual final point, but in the present state of 

 our knowledge it would be unwise to exclude the possibility of an 

 earlier occurrence in some cases. As instances such hermaphrodite 

 animals as Tunicata and leeches may be cited. From Julin's re- 

 searches ') on the oogenesis and spermatogenesis of Styelopsis grossularia 

 it would seem to follow, that the separation of two categories of germ- 

 cells, the forerunners of female-oocytes and of spermatocytes, must 

 be relegated to an earlier epoch than the final division prior to the 



1) As Mevbs' researches (to be subsequently referred to) prove, 

 the like is also true of the diagram of spermatogenesis, at any rate in 

 certain cases, such as Paludina and Pygaera. 



2) in: Bull. sc. Prance, p. 25, 1893. 



