July 4, 1913] 



SCIENCE 



31 



spermatogonia. Two of these, undoubtedly 

 the accessories, are oval in shape and some- 

 what larger than the others, which are rod- 



In the primary spermatocji;es ten chromo- 

 somes appear in the late prophase of division, 

 eight large bivalents plus the two unpaired ac- 

 cessories. In the metaphase of this division 

 the accessories pass to one pole, undivided, and 

 in advance of the other chromosomes. Thus, 

 this division which is evidently the reduction 

 division gives rise to two different types of 

 secondary spermatocytes. The one type con- 

 tains eight ordinary chromosomes or auto- 

 somes, and the other eight autosomes plus the 

 two accessories. 



In the late prophase and early metaphase of 

 division in the secondary spermatocyte four 

 large chromosomes appear in the one type of 

 cell and four plus the two accessories in the 

 other. Thus a second fusion of the autosomes 

 in pairs has evidently taken place. This is not 

 to he looked on as a second reduction division, 

 however, as the autosomes in the late meta- 

 phase of division in these cells manifest their 

 bivalent nature again. The secondary sperma- 

 tocytes containing the four large chromosomes 

 give rise to two spermatids each with four bi- 

 valents or eight univalents ; and those contain- 

 ing the four large chromosomes and the two 

 accessories give rise to spermatids containing 

 four bivalents or eight univalent chromosomes 

 plus the two accessories which have divided 

 here for the first time since the last sperma- 

 togonial division. 



The spermatids transform directly into 

 spermatozoa. The conspicuous centrosome 

 emerges from the sphere and divides into two 

 parts which for some time remain connected 

 by a thick strand of material. The anterior 

 centrosome comes in contact with the nuclear 

 wall while the posterior one becomes trans- 

 formed into a ring through which extends the 

 developing axial filament. In the meantime 

 the sphere migrates around the nucleus to a 

 point opposite the anterior centrosomes where 

 it becomes fixed as the acrosome. Long before 

 the axial filament is fully developed the pos- 

 terior ring-shaped centrosome is thrown off 



and lies in the cytoplasm away from the axial 

 filament. During the final development of the 

 spermatozoan a large mass of cytoplasm con- 

 taining the posterior centrosome is thrown off 

 by the cell. Careful measurements of a large 

 number of the mature spermatozoa show that 

 they are of two distinct types, one being much 

 larger than the other. 



The investigation was extended to studies 

 of the germinal and somatic cells of both male 

 and female pig embryos. It was again found 

 in the male that the spermatogonial number of 

 chromosomes is eighteen and that the same 

 number prevails in the somatic cells, two of the 

 chromosomes being somewhat larger. Twenty 

 chromosomes, four somewhat larger and evi- 

 dently the accessories, were found in the 

 oogonia, and the same number prevails in the 

 somatic cells of the female. It is evident that 

 the eggs containing the reduced number of 

 chromosomes, which is ten, when fertilized by 

 the one type of spermatozoan containing ten 

 chromosomes give rise to individuals contain- 

 ing twenty chromosomes in their cells, or fe- 

 males ; while those fertilized by the other type 

 containing only eight chromosomes give rise 

 to individuals with eighteen chromosomes in 

 their cells, which was found to be the number 

 in the male. 



Many investigators have found a similar 

 dimorphic condition in the number of chromo- 

 somes in the two sexes of some of the inverte-' 

 brates; and although the same condition was 

 predicted to exist in the vertebrates possessing 

 the X-element, it has, heretofore, never been 

 actually shown. 



N. E. Jordan in a recent abstract' says that 

 the heterochromosomes are unquestionably 

 lacking in the pig and several other mammals. 

 Since the appearance of this article I have 

 carefully reinvestigated my material and am 

 thoroughly convinced that my conclusion is cor- 

 rect. A detailed account of this investigation 

 will be published later. 



j. e. wodsedalek 

 Zoological Laboeatoet, 

 Univeesity op Wisconsin, 

 January 30, 1913 

 'Science, N. S., Vol. XXXVII., Nc 946, pp. 

 270-271. 



