THE EMBRYOLOGY O.F THE OPOSSUM 



17 



which is from 0.0006 to 0.004 mm. in thickness, fits closely 

 about the egg cell and the first polar body and presses them 

 tightly together, so that the latter makes an elongated de- 

 pression on the surface of the egg. Beneath the zona pellucida 

 a radially striated layer some 0.0012 mm. thick is visible in 

 some preparations. Hartman ('16) noticed it, but considered 

 its homology to the zona radiata of other forms uncertain. 



The testicular sperm. The spermatogonium (Painter, '22) 

 has twenty-two chromosomes (fig. 2, A), including ten pairs 

 of autosomes, an X-chromosome, and a Y-chromosome. The 

 last two, the sex chromosomes, are visibly represented during 



Fig. 2 Spermatogenesis (after Painter, '22). A, polar view of spermato- 

 gonium showing ten pairs of autosomes, an X chromosome, and a Y-chromosome. 

 B, equatorial view of first maturation division showing 12 chromosomes on account 

 of precocious separation of X and Y elements. C, equatorial view of second 

 maturation division showing X-chromosomes. Not all chromosomes are included 

 in this figure. D, equatorial view of second maturation division showing 

 Y-chromosomes. The true haploid number (11) may be counted at the upper pole. 



the growth stage of the primary spermatocyte as the 

 chromatin nucleolus (Painter, '24), which is the product of 

 their synapsis. The X and Y components segregate (fig. 2, B) 

 in the first maturation division, so that two kinds of secondary 

 spermatocytes are formed in equal numbers. Both Jordan 

 ('11) and Painter ('22), have recognized a distinct resting 

 stage between the two maturation divisions. In the second 

 maturation division the sex chromosomes divide equationally 

 (fig. 2, C and D). 



Spermiogenesis (fig. 3) has been described by Duesberg 

 ('20). Fat droplets are always present in early spermatids. 



