spermatogenesis of the Mongoose, etc. 171 



synapsis is a variable factor in different species" (Kirillow, p. 145); that is, 

 synapsis (contraction phase) is regarded as a normal phase in the auxocytes 

 of some forms (species), but is thought not to appear in others. 



MULE. 



The presence of an accessory chromosome in the mule was also noted 

 briefly in an earlier paper (Jordan, 1911, 1912). Here it usually appears 

 double in pre- and post-synaptic stages (figs. 34 and 36). At synapsis it 

 appears as a single large oval chromatic body (fig. 35). It is very clearly 

 recognizable among the prophase chromosomes as a large paired element 

 (fig- 37)- Since at certain phases (synapsis) it may be a single compact 

 body, it is perhaps more legitimate to regard it as a double X-element. 



In the case of the mule it will be impossible further to trace this body 

 with any degree of certainty. As first noted by myself (see quotation 

 from my unpublished manuscript in a paper by Dr. R. H. Whitehead, "A 

 Peculiar Case of Cryporchism, and its Bearing upon the Problem of the 

 Function of the Interstitial Cell of the Testis," Anat. Record, Aug. 1908; 

 vol. II, No. 5), spermatogenesis in the mule does not ordinarily proceed 

 beyond the early prophase. This is the cause of the sterility of mules bred 

 inter se; the mule does not generally produce ripe spermatozoa. My obser- 

 vations were subsequently confirmed by Poll (1911), who succeeded in 

 observing a few first maturation spindles at metaphase. 



BULL. 



The spermatogenesis of the bull has been fully described by Schoenfeld 

 (1901). My observations agree essentially with his illustrations. Schoen- 

 feld figures a stage similar to my illustration of postsynapsis (fig. 38), where 

 a single large, oval, chromatic " accessory chromosome " appears among the 

 bivalent threads, close to the idiosome. This element, at this as well as 

 at earlier and later stages, Schoenfeld describes as the "corpuscle intra- 

 nucleare." In the light of what we now know of "sex-chromosomes" 

 generally, and in mammals more particularly, this body may, I believe, be 

 regarded confidently as an accessory chromosome. 



DOG. 



Figure 39 illustrates a resting primary spermatocyte with pale plasmo- 

 some and chromatic, bilobed chromosome nucleolus. No very satisfactory 

 synapsis stages could be observed in my material.^ Figures 40 and 41 

 illustrate two successive postsynaptic phases in which the X-element is 

 conspicuously present as a chromatic, sharply contoured, irregularly oval 

 body close to the idiosome. We are probably here again dealing with an 

 accessory chromosome. The attempt further to trace this body, however, 

 was very disconcerting by reason of the absence of definite marks of identity, 

 lack of clear delimitation of the relatively large number of small chromo- 

 somes, and confusing contradictions of trying observations. In short, any 

 further analysis was unsuccessful. 



• That is, from the standpoint of a conspicuous " X-element." 



