57 



P.6.G. 



- O0.C ■ ""ai pnases m aerermmwion m k 



A /x A 



^B99 'Segg. 



^egg. 



Fig. 4. — Oogenesis and determination of sex in 

 litija bttfts. 



Flc. 5. — Probable course of spermatogenesis 

 in Piittidiiiti (after the statements of 

 F. Meves). 



Diagram 4 will help to make clear the determination : 

 P.G.C. is a primary germ-cell, it divides and becomes two 

 secondary germ-cells; the products of the secondary germ-cell 

 destined to be male eggs pass through one division more than 

 those producing female eggs, as indicated on the lines marked 

 O.C. The lower part of the diagram refers to the ripening of 

 the egg, preparing it for the reception of the sperrriatozoon. At 

 the first division half its chromatin is cast out as a polar body 

 — the small round circle ; the chromatin again divides and casts 

 out the second polar body, at the same time the first polar 

 body divides, thus giving one functional egg and three function- 

 less ones. 



I must here point out that Beard's research only refers to 

 the morphological determination of sex, and in no way attempts 

 to explain its origin. 



Diagram 5 indicates, according to Beard, the probable 

 course of spermatogenesis and the determination of the two 

 sorts of spermatozoa in Paludina and other organisms known 

 to possess two sorts of male gametes. 



