ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 423 



chromosomes and their individuality, and finds it lacking in scientific 

 security. It seems to him that in every animal there is a peculiar 

 manoeuvring of the chromatin, adapted to the physico-chemical 

 exigencies of the case. As regards inheritance, we only see the gross 

 manoeuvres of regiments, so to speak, and we cannot argue from that to 

 the behaviour of individual combatants. 



Formation of Yolk in Egg of Sparrow.* — Dubuisson finds that just 

 before the formation of the vitellus begins, the central zone of protoplasm, 

 originally homogeneous, becomes vacuolar. The periphery remains an 

 annulus of granular protoplasm. The nucleus lies tangentially to this 

 annulus, quite eccentrically. Yolk plates begin to be deposited peri- 

 pherally, and continue to appear in centripetal order in concentric 

 layers. Different types of yolk-plates are described, and account for the 

 old distinction between " white yolk " and " yellow yolk." The yellow 

 yolk is a more evolved condition of the white yolk. There is also a less 

 important centrifugal formation of yolk. 



New Theory of Sex-Production.f — E. B. Wilson discusses the 

 theory recently developed by R. Hertwig,J which is antagonistic to 

 the view that sex is already determined in the fertilised egg. Issakowitsch 

 has shown that in Simocephalus sex production shows a definite relation 

 to temperature changes. At 24° C. there is a continuous production of 

 parthenogenetic females, with only an occasional male ; a reduction to 

 16° C. quickly leads, and a reduction to 8° C. immediately leads to the 

 appearance of males and to the production of winter eggs. Starvation 

 brings about the same result. Yon Malsen has shown that in Dinophttus 

 an elevation of temperature from 10°-12° C. to 25° C. changes the ratio 

 of male and female eggs from 1:3 to 1 : 1 • 75 or even 1:1. R. Hertwig, 

 working with frogs, was inclined to think that higher temperatures 

 favoured the production of females, and found that over-ripeness or 

 under-ripeness of the eggs led in every case to a large excess of males. 

 In one case of under-ripe eggs 40 larvas successfully reared were all 

 males. 



Hertwig's general theory is that the ratio between the nuclear and 



the protoplasmic mass ( - J tends towards a normal value that is in the 



long run constant for the species, though it undergoes cyclical changes 

 both in the individual cell and in successive generations of cells. In 



ordinary or " functional " growth the value of - decreases, in the sub- 



p 



sequent " divisional " growth it rises above the normal, in cell-division 

 the normal ratio is approximately restored. In long continued vegeta- 

 tive or asexual reproduction there is a gradual permanent increase in the 



Jc 

 value of -, a nuclear hypertrophy, which is remedied by conjugation. 



In the male - is assumed to have a higher value than in the female, and 

 V 



* Comptes Rendus, cxli. (1905) pp. 776-7. 



t Science, xxiii. (1906) pp. 189-91. J Verh. Deutsch. Zool. Ges., 1905. 



