The determination of sex in animal development. 731 



not only for a female embryo and a set of female-eggs, but also for 

 a new batch of male-eggs. 



In this way the male-eggs, like the Metazoan individuals (Weis- 

 mann), are on their way to a cul-de-sac, out of which part of their 

 progeny, the spermatozoa, can only extricate themselves by union with 

 another egg, male or female in destination. The real responsibility 

 for the continuance of the species thus falls upon the females. 



An attempt must be made to indicate where in the life-cycle the 

 differentiation into male- and fernale-eggs must be conceived as taking 

 place. As unlike gametes, which, however, are incapable of conjugation 

 even with the corresponding unlike gametes of another female in- 

 dividual, their direct forerunners must be differentiated, as in 

 certain Protozoa^) in the final, or in one of the last divisions of 

 BovERi's oogonia, i. e., just prior to the period, where these become 

 oocytes in Boveri's sense. To put the matter differently, Boveri's 

 oocytes must be differentiated into the forerunners of male-eggs and 

 female-eggs, or into two sorts of oocytes, respectively destined to 

 give rise to male and female organisms (Fig. A). It may be very 



1) The fact is not forgotten, that in the Protozoa the divisions 

 here referred to as preceding conjugation do not yield unlike gametes. 

 To take a well-known instance, that of Paranioeciiim according to 

 Maupas' diagram, in P. caudatum the micro-nucleus in each conjugating 

 individual divides twice, forming four nuclei. Of these three are 

 abortive, while one divides once more into the stationary and the 

 wandering nucleus. This latter is not, I take it, to be looked upon as 

 representing in any way a division into what might become unlike 

 gametes in the sense of those of the Metazoa ; sex not being really 

 represented here. The exchange is one of equivalent gametes, comparable, 

 if at all so, to what happens in the reciprocal hermaphroditism of the 

 snail, or, better, to instances of parthenogenesis. If unlike gametes 

 here arose — gametes of four kinds — their formation would date 

 from the first two divisions of the micronucleus. The so-called "polar 

 bodies" — the three abortive nuclei arising from these two divisions 

 — are, strictly speaking, not comparable to the abortive cells of the 

 like name in Metazoan oogenesis, but are similar to abortive forerunners 

 of gametes, as an example, to abortive spermatocytes. A differentiation 

 of unlike gametes, however, is witnessed in Vorticella, according to the 

 researches of Maupas. His diagram, showing the formation of the 

 forerunners of the micro-gametes, reveals an additional mitosis in the 

 individual about to give rise to the forerunners of the latter, in fact, 

 this individual divides into two, and each of these undergoes the 

 before-mentioned changes in its micronucleus. 



Zool. Jahrb. XVI. Abth. f. Morph. ^g 



