294 



SCIENCE. 



[N. S. Vol. XXIV. No. 610. 



and all the anthropoid apes; the brain of 

 the manatee; the brain of a sheep with 

 defective callosum and fornix, and that of 

 a eat lacking these parts altogether; that 

 of a child born at term with the cerebral 

 hemispheres in the fetal condition of large 

 cavities and thin parietes; an adult hnman 

 brain with an extra precommissure ; the 

 brains of two philosophers and mathemati- 

 cians differing markedly, and that of a 

 mulatto resembling one of the former; the 

 brain of a dentist presenting two self-in- 

 flicted pistol-ball wounds; the separable 

 dental laminae of a young elephant ; masto- 

 don teeth and bones from near Ithaca; 

 the brains of some rare Japanese sharks. 



Sex Differentiation in Dinophilus: Edwin 



G. CoNKLiN, University of Pennsylvania, 



Philadelphia. 



Korschelt (1882) discovered that Dino- 

 philus apatris lays two kinds of eggs, the 

 one three times the diameter of the other; 

 and he further determined the important 

 fact that the small eggs invariably develop 

 into males and the large ones into females. 

 He did not, however, study the oogenesis 

 of these two kinds of eggs. The occurrence 

 some years ago of a related species of 

 Dinophilus in the marine aquaria at the 

 University of Pennsylvania gave me an 

 opportunity to study this problem. 



The earliest stages in the developing 

 ovary of Dinophilus which I have ex- 

 amined shows only one kind of primitive 

 egg cells or oogonia. These cells are very 

 small and details of their structure are not 

 readily made out, but all are approxi- 

 mately of the same size. The size and 

 structure of these cells and of their nuclei 

 are apparently the same in all cases. I 

 have repeatedly seen the last oogonial 

 division, but the chromosomes are so small 

 and numerous (about 20) that it has not 

 been possible to distinguish any constant 



difference either in the number or in the 

 shapes of these chromosomes. 



After the oogonia have entered upon the 

 growth period and have reached about 

 twice their original diameter they begin to 

 fuse together and in this process at least 

 twenty-five or thirty fuse to form the fe- 

 male eggs, while a much smaller number 

 fuse to form the male eggs, a fact recently 

 reported by R. Hertwig (1905). In the 

 fusion, the cell boundaries first disappear 

 and then some time later the nuclear mem- 

 branes dissolve and the nuclear contents are 

 scattered in the cell, where they slowly dis- 

 solve. Only one nucleus remains in the 

 syncytium; this is in most cases centrally 

 placed, but I have been unable to deter- 

 mine whether its survival depends upon its 

 position or upon some intrinsic difference. 



In the fully formed ovarian eggs the 

 nucleus of the small eggs is relatively larger 

 than that of the large ones, as E.. Hertwig 

 (1905) has stated, the relative diameter of 

 the nucleus to the entire cell beirtg about 

 1 : 4 in the case of the male eggs and about 

 1 : 6 in the case of the female eggs. In 

 this fact Hertwig thinks we have the cause 

 of sex determination. It is doubtful 

 whether this can be accepted as a general 

 explanation, and even in the case of Dino- 

 philus it is not certain that it is the true 

 explanation. The relatively greater size 

 of the nucleus in the male eggs is probably 

 due to the smaller number of cells which 

 fuse to form the cell body, and this may 

 result from the relative isolation of the 

 male eggs as compared with the crowding 

 of the female eggs. 



In the first maturation division the chro- 

 mosomes, though small, may be counted, 

 and it is found that there are ten in both 

 male and female eggs. No differences in 

 the size of individual chromosomes can be 

 detected in the two kinds of eggs. 



The manner in which the primary oocytes 



