1902.] NICHOLS — SPERMATOGENESIS OXISCUS ASELLUS LINN. lOl 



2. The Earlier Stages in the Development of the Germ Cells 



IN Crustacea, with Especial Reference to the 



Problem of Reduction. 



a. Review. 

 Decapoda. 



1878. Grobben gives almost no figures of the earlier stages and 

 does not consider the subject in detail. 



1884. Nussbaum (Astacus fluviatilis) does not distinguish between 

 spermatogonia and spermatocytes. Five figures of mitoses are given 

 in which the chromosomes are shown to be spherical at the begin- 

 ning of the metaphase, but they soon elongate to a rod-like shape. 



1885. Carnoy studied among the Decapods, Astacus fluviatilis, 

 Crangon vulgaris and several species of Brachyura and Anomura. 

 In no case are more than thirteen figures given. It is impossible to 

 determine in every case the generation to which the cells belong. 

 The mode of origin of the chromosomes is not fully traced, and it 

 is impossible to determine with accuracy, therefore, anything with 

 regard to the question of reduction. In the case of Astacus, as far 

 as can be judged from the figures given (Figs. 246^, b, c, d, e and 

 f), the division is transverse. The mitosis figured occurred in 



August, and, according to vom Rath, it is from this month until 

 December that the final divisions of the spermatogonia and those 

 of tlie spermatocytes take place. A transverse spermatogonic 

 division is improbable. The chromosomes are shown to arise, 

 however, through the shortening and thickening of rods, resulting 

 from the breaking up of the nuclear network. The transverse 

 division, if it be such, is therefore probably that of the first sperma- 

 tocyte. The same is perhaps true of Crangon cataphractus (Figs. 

 247 and 248). Of peculiar interest is the constitution of the chro- 

 mosomes of Crangon cataphractus, as shown in Figs. 249^, b, c, d, 

 PI. VII. According to these a chromosome in longitudinal view 

 consists of a double row of from three to five granules. A recon- 

 struction of the chromosome from these figures leads to the concep- 

 tion of a rod split longitudinally several times. 



Cytoplasmic Structures. — A dense mass, lying within the cyto- 

 plasm during the prophases and migrating to the poles of the spin- 

 dle as it is formed, is shown for Crangon. No centrosome is 

 figured as lying within this mass, to which the name *'Nebenkern" 

 is given. The same name is applied to a body lying in the cyto- 



