6 AMERICAN JOURNAL OF VETERINARY MEDICINE 



located in the more or less elongated head. 

 The tail which is surrounded by a thin mem- 

 brane (tail envelope) is an organ of locomo- 

 tion and is usually broken off as the sperm 

 enters the egg (Fig. 12), its function having 

 been completed. 



VIII. The Ripening of the Female 

 Germ Cell. The development of the female 



- cell dififers only in minor details from that of 

 the male. In ripening, however, each division 

 produces daughter cells extremely different in 

 size. The larger ones (Figs. 18 and 16) 

 continue to mature and prepare for fertiliza- 

 tion, while the smaller (the polar bodies. Fig. 

 17) die and are absorbed. It is readily seen 

 from the plate that, from the same number 

 of green-cells, four times as many sperms 

 are produced as there arc eggs. This is very 

 fortunate as the sperms must undergo much 

 more hazardous experiences in reaching their 

 desired destination, and thus more liable to 

 destruction than the eggs. 



IX. Union or the Sperm and the Egg. 

 After the sperm enters the egg (Figs. 12 to 

 15), the middle piece, which carries "the ac- 

 tive center of cell division," the centrosome, 

 turns in the direction of the egg nucleus (Fig. 

 12) and a joint spindle between the male and 

 female cells is formed (Figs. 13, 14 and 15). 

 The most important consideration in this proc- 

 ess is the fact that the reduced number of 

 chromosomes brought in by each parent cell 

 now unite to form the normal number for the 

 offspring (Fig. 15). Furthermore, the chro- 

 mosomes from each parent are equally dis- 

 tributed in every daughter cell (study Figs. 

 13, 14 and 15). 



X. Sperm Formation in the Domestic 

 Animals. All the essential steps in the for- 

 mation of the sperms in the common domes- 

 tic animals, conform in general to the fore- 

 going consideration. It thus becomes neces- 

 sary to refer only to the variations and es- 

 pecially interesting points in the germ cells 

 of these animals. 



A. Birds. The cells of the birds are very 

 small, and the constituent parts are so crowd- 

 ed and irregular in shape, that a study of 

 these forms is very laborious. Guyer, for ex- 

 ample, has only recently (Oct., 1916) pub- 

 lished the results of over ten years' study on 

 the sperm formation in the common rooster. 

 Here he finds, in addition to the mass of ir- 

 regular chromosomes, a larged curved sex- 

 chromosome, which stands aloof from its 

 neighbors while on the first maturation spin- 

 dle. "A similarly constant element, differing 

 in form from the common fowl is found in 

 the guinea and in the guinea chicken hybrid." 

 Since the early germ cells of the chicken 

 have sixteen ordinary chromosomes plus this 

 sex-element, and since this element neither 

 conjugates in the ripening" process nor di- 

 vides in the first ripening division, one daugh- 



ter cell has eight ordinary chromosomes, while 

 the other possesses, in addition to these, the 

 large curved sex-element. The ordinary 

 chromosomes of each daughter cell now pair 

 and fuse, leaving four in one and four plus 

 the sex determiner in the other. All chromo- 

 somes again divide in the second division, 

 and thus it is brought about that two classes 

 of cells are formed containing 4 and 5 

 chromosomes respectively. 



In the hybrid pheasants and hybrid guinea- 

 chickens the normal development continues 

 up to synapsis when the chromatin fibers are 

 unable to unravel from the synaptic knot (see 

 Figs. 4 and 5). The mass of chromatin re- 

 mains attached to the nuclear wall and then 

 either disintegrates, or abnormal spermatozoa 

 are formed which are incapable of fertiliza- 

 tion. This clearly accounts for the fact that 

 the breeding of hybrid birds of various kinds 

 is an impossibility. 



B. Mammals. One of the first mammals 

 to be studied was man. Although perhaps not 

 classed as a domestic animal, the "close rela- 

 tionship" would warrant a brief reference. 

 Man represents one of the most problematical 

 cases of cell study. The number of somatic 

 chromosomes has been variously counted from 

 24 to 38 (Fleming 24, Farner, Moore, Walk- 

 er 32, Wieman 33 to 38), while the number 

 in the testes are given as 47 by Winiwarter 

 and 22 by Guyer and Montgomery. The first 

 worker obtained his data from a white man, 

 while Gu\er and Montgomerj- worked on ne- 

 gro material. In the case of Winiwarter's 

 work on the germ cells of the white man, 

 the numbers in the maturation divisions (Figs. 

 7, 8 and 9) are 23 and 24, while Guyer shows 

 that the corresponding numbers in the negro 

 are 10 and 12. If the works of the last two 

 investigators stand the test of future re- 

 searches, we will have a very definite and 

 substantial criterion by which we can segre- 

 gate the white and black races. No doubt, 

 it soon will be a very simple matter to take a 

 small bit of muscle, connective tissue or even 

 nasal epithelium from any individual, count 

 the chromosomes in the dividing cells and 

 determine the race of that individual. Hoy 

 (1916) suggests that the variation in the num- 

 ber of chromosomes in the different individ- 

 uals studied might represent intermediate hy- 

 brids between the white and black races. In 

 this case, the percentage of negro blood flow- 

 ing in any man's veins, would be in direct 

 proportion to the number of chromosomes in 

 the cells of his body. 



Furthermore, Guyer finds in man two sex- 

 chromosomes which do not associate with the 

 other elements on the first maturation spindle 

 (Fig. 7) but pass bodi'y to one pole, thus 

 giving 10 chromosomes in one daughter cell 

 and 12 in the other. The ten common chro- 

 mosomes now unite two by two, giving second 



