The Spermatogenesis in Pentatonaa up to the Formation of the Spermatid. ^5 



the size of the latter being variable. A true nucleolus (n) is present, 

 but no other form of nucleolar structure. 



The commencement of the prophase of mitosis (Fig. 5, and the 

 upper cell in Fig. 14) is marked by a change in the arrangement of 

 the chromatin. The microsomes of the nuclear reticulum gradually 

 lose their separateness, and, apparently owing to a process of flowing, 

 connect together so as to form continuous threads of chromatin. Thus 

 is gradually reached the state corresponding to the "dense spirem" 

 stage, but the threads of chromatin are so slender and numerous that 

 it is as yet impossible to determine whether or no the whole chromatin 

 mass forms a single thread (Figs. 6—8). These threads shorten and 

 thicken, their contours become more even and the greater number of 

 the loops become arranged close to the nuclear membrane (Figs. 9 — 11). 

 This is the "loose spirem" stage; and in one particularly favorable 

 nucleus (Fig. 10), the whole of which lay in the plane of the section, 

 I could clearly determine that the chromatin formed a single, conti- 

 nuous thread, with two free ends. When a nucleus in the loose 

 spirem stage is seen from one surface only (Fig. 7), the chromatin 

 appears in the form of a number of more or less parallel threads. 

 The spirem stages, particularly the loose spirem, are not often seen, 

 and hence are probably passed through with relative rapidity. 



The next stage of the spermatogonic prophase is marked by 

 further thickening and transverse segmentation of the chromatin thread 

 (Figs. 12—14), together with an increase in the volume of the nucleus. 

 The spirem thread segments into 14 chromosomes which are at first 

 irregular in form, and to some extent in size. The spirem thread does 

 not segment at all points simultaneously; there is no longitudinal 

 division of the chromosomes before the stage of metakinesis. The 

 chromosomes are at first irregularly elongated and bent in form. They 

 gradually shorten and thicken (Figs. 15, 23, 24), their contours becom- 

 ing more even, until the definitive form of a nearly spherical oval is 

 attained (Figs. 19 — 22). These segmentation stages occur abundantly, 

 and probably occupy considerable time. 



During the preceding stage the centrosome first appears as a 

 clear though small granule lying within the substance of the idozome 

 (Fig. 17 c); it stains a deep red with satfranine. The point may be 

 noted that my figures represent the centrosome slightly larger than 

 it really is, but though small it can be clearly seen and its position 

 accurately marked with the camera lucida. Before the chromosomes 

 have acquired their definitive form the division of the centrosome 



