446 



HISTOLOGY 



spermatocyte now passes through a growth period during which it in- 

 creases somewhat in size (Figs. 7-15). 



The growth period presents various stages of intense activity and great 

 protoplasmic alterations. After a brief resting stage (Fig. 408, 7) it enters 

 upon a pre-synaptic phase, during which the nuclear reticulum becomes 

 slightly chromatic and disposes itself into a lattice-work arrangement 

 (Fig. 408, 8). At the same time the accessory chromosome lengthens 

 out into a club-shaped mass extending through almost the entire diameter 

 of the nucleus and becoming attached at its lesser end to the nuclear 

 spireme. The spireme now breaks up into a number of segments 

 (approximately 34) and these form loops at one pole of the nucleus (Fig. 

 408, 9). This represents synizesis. Subsequently these loops open 

 up and one end becomes free (Fig. 10). The segments now unite in 

 pairs by their free ends (Fig. n) to form half the number of original 

 loops. This stage is synapsis and, according to Montgomery and others, 

 represents a pairing of homologous paternal and maternal chromosomes. 



FIG. 409. Aplopus Mayeri. Four stages in postsynapsis, showing postsynaptic reticulum 

 and closing up of the elongate and ring-shaped accessory chromosome. X 1400. (Drawn 

 by H. E. JORDAN.) 



The large loops are the bivalent postsynaptic chromosomes. This 

 end-to-end union is an example of telosynapsis. Meanwhile the acces- 

 sory chromosome assumes a position to one side of and usually beneath 

 the loops. Its longitudinal split opens up more or less completely during 

 this stage. 



During subsequent postsynaptic stages the chromosomes again 

 arrange themselves into a reticulum (frequently giving indications of a 

 longitudinal split) in the shape of a lattice-work. The accessory chro- 

 mosome, meanwhile, closes up and shortens down into a compact, deep 

 staining body closely applied to the nuclear wall (Fig. 409, 12-15). 

 During the ensuing prophases (Fig. 410, 16-18) the spireme becomes 

 split into a number of segments (17), each of which presently undergoes 

 first a longitudinal, and secondarily a transverse fission, to form typical 

 tetrads of various forms and sizes. While the ordinary chromosomes 

 are at this stage, and still pale staining, the accessory chromosome is 

 readily distinguishable as a body of deep staining capacity and sharp 

 contour. It varies much in form, being bipartite, quadripartite, or 

 U-shaped. A later prophase (Fig. 410, 19) shows all the chromosomes 



