16 TEXT-BOOK OF EMBRYOLOGY. 



the lumen of the tubule. When fully developed they become detached 

 from the Sertoli cells and lie free in the lumen of the tubule. 



The work done within the past decade on spermatogenesis in the human 

 has established the relation of chromosome behavior here to that in the 

 lower animals, showing some interesting coincidences. In the last of several 

 studies by different investigators, Wieman has critically observed conditions 

 in both the white and the negro. In division of the spermatogonium 24 

 chromosomes appear, two of which are designated idiochromosomes (XY 

 pair). During the period of growth to a primary spermatocyte the XY pair 

 persists as a deeply staining bipartite body (Fig. 9, a). In the prophase of 

 primary spermatocyte division pairing or synapsis results in 12 bivalent 

 chromosomes, the XY pair retaining its identity (Fig. 9, V). When meta- 

 kinesis occurs the XY element divides lengthwise, but whether the other 1 1 

 divide lengthwise or transversely has not been determined (Fig. 9, c). In 

 division of the secondary spermatocyte the n chromosomes divide, each 

 giving one-half of itself to a spermatid; but the XY element gives X to one 

 spermatid and Y to the other. The result of this chromosomal behavior is, 

 therefore, that the usual reduction in number is accomplished but that the 

 spermatids, and hence the spermatozoa, are of two classes differing as to the 

 X and Y chromatin content. 



MATURATION OF THE OVUM. 



The female germ cell, before it is fertilized, goes through a process of 

 maturation similar to that of the male germ cell. The result is essentially 

 the same: the mature ovum contains a reduced number of chromosomes. 

 There is this difference, however, that while the chromatin elements are 

 distributed equally during the reduction divisions, one cell alone retains 

 practically all the cytoplasm and deutoplasm present in the primary oocyte. 

 This cell becomes the functional ovum while the other cells are pinched off as 

 minute bodies, containing but little of the cytoplasm, which are known as 

 polar bodies and eventually degenerate and disappear. 



The early maturation stages of the female sex cell are very similar to 

 those of the male. The oogonia contain the diploid number of chromosomes 

 and divide by ordinary mitosis. After several generations they pass through 

 a period of growth and are then known as primary oocytes. During the 

 growth period there occurs a condensation of the chromatin, and synapsis 

 of the chromosomes probably takes place at this time. The nucleus then 

 resumes its reticular structure. Following this the spireme is formed, 

 preparatory to division, and segments into the haploid number of chromo- 

 somes. From this stage the process varies somewhat in different animals. 



