36 STUDIES IN SPERM ATOGENESIS. 



many good equatorial plates from egg follicles (fig. 20) shows 30 large 

 chromosomes, indicating an equal pair in place of the unequal pair of 

 the male. 



Chelymorpha argus (Family Chrysomelidae). 



This species was found in larval and adult stages on Convolvulus 

 arvensis at Harpswell, Maine, in July and August. It shows the 

 same conditions as Trirhabda and Tenebrio, so far as the unequal pair 

 of chromosomes is concerned, and is especially favorable for study 

 of synapsis stages. The number of chromosomes in the spermatogo- 

 nia (plate ix, fig. 36) is 22. Here the components of the unequal 

 pair are the small spherical chromosome and one of the several chro- 

 mosomes third in size, forming a comparatively small unsymmetrical 

 bivalent (figs. 47-49). The spermatogonia occupy the outer end 

 of each follicle, and next to them comes a layer of cysts in which the 

 chromosomes from the last spermatogonial division are closely massed 

 in the form of short deeply staining loops at one side of the nuclear 

 space (fig. 37). Following this synizesis stage comes one in which 

 some of the short loops have straightened, their free ends extending 

 out into the nuclear space (figs. 38 and 39). Figure 40 shows the 

 nucleus of a slightly later stage in which the free ends of two straight- 

 ened chromosomes are on the point of uniting. In figures 41 and 42 

 the point of union of homologous chromosomes is indicated in some , 

 cases by a knob, in others by a sharply acute angle. In a slightly 

 later stage (fig. 43), when all of the short loops have straightened and 

 united in pairs, the point of union is no longer visible, all of the loops 

 being rounded at the bend and of equal thickness throughout. My 

 attention was first called to this method of synapsis by the con- 

 spicuous difference in number and length of loops in the synizesis 

 stage compared with the later bouquet stage just before the spireme is 

 formed. Following the synapsis stage shown in figure 43 comes one 

 in which the loops lose their polarized arrangement and unite to form 

 a continuous spireme (figs. 44 and 45). In this form, the heterochro- 

 mosome pair could not be distinguished until the spireme stage, 

 and it is, therefore, uncertain whether these chromosomes remain 

 condensed after the last spermatogonial divisions and are hidden 

 among the massed and deeply staining loops of the synizesis and syn- 

 apsis stages, or whether they pass through the same synaptic phases 

 as the other chromosomes, condensing and remaining isolated at the 

 beginning of the spireme stage. An early prophase of the first matu- 

 ration mitosis (fig. 46) shows segments of the spireme longitudinally 

 split, and in some cases transformed into crosses which show a trans- 

 verse division also. Most of the equal bivaleuts have the dumb-bell 



