170 Papers from the Marine Biological Laboratory at Tortugas. 



pair may entirely and widely separate, as is the case in the mule for example. 

 The double nature of the heterochromosome suggests a pair of idiochro- 

 mosomes; but the fact that during and immediately after synapsis this 

 element appears single invalidates somewhat such an assumption, and 

 suggests rather a double accessory chromosome, or X-element of Wilson. 

 In leptotene and diplotene phases it very frequently lies in a clear space 

 free of threads, a condition similar to that described by Wilson (191 2) for 

 Largus and Oncopeltus. 



Figure 27a would seem to permit of not the slightest doubt of the presence 

 of a "sex chromosome" (Wilson) in the white mouse. Its later history, 

 however, is still obscure. Division figures are very abundant in my material 

 and the preservation is nothing short of superb, but at no later stage can I 

 with certainty identify this body. Occasionally a spindle appears with a 

 large chromosome in advance of the main metaphase group (fig. 276) , but 

 the instances seem too rare to have much significance apart from morpho- 

 logical marks of identification from among the chromosomes of the meta- 

 phase plates. 



SHEEP. 



In sheep I am able to distinguish the "X-element" during the presyn- 

 aptic (fig. 28), synaptic (fig. 29), and postsynaptic (fig. 30) stages. Both 

 in the first and second stages it is usually attached to the chromatic spireme, 

 or one of its segments. During the postsynaptic stages it is usually split 

 or bilobed, and, as at earlier stages, frequently but not invariably lies close 

 to the idiosome. Synapsis is effected by side-by-side union. In the sheep, 

 also, this accessory chromosome eludes later certain identification. 



HORSE. 



As previously briefly noted (Jordan, 191 1, 1912), during postsynaptic 

 stages the horse shows frequently a tripartite X-element (fig. 33), usually 

 next the idiosome. In the resting phase of the primary spermatocyte 

 (fig. 31), and during synapsis, it appears double. Further clear tracing of 

 this body seems impossible. The evidence is thus far from sufficient to 

 warrant even an inference as to whether we are here dealing with a multiple 

 X-element (as in Sinea, for example, according to Payne, 1909) or a pair of 

 idiochromosomes. Both conditions have been described for mammals; 

 for example, man as having a double X-element (as in Syromastes — ^Wilson, 

 1909) by Guyer (1910), and guinea-pig with a pair of heterochromosomes, by 

 Miss Stevens (1912). The only point sought to establish is the presence of 

 "sex chromosomes" of some sort. 



Kirillow (1912) has recently published the first number of his projected 

 studies on the spermatogenesis of the horse. The accompanying illustra- 

 tions, including all the chief stages, give no indication of a heterotropic 

 body so conspicuous in my preparations; nor is any mention made of such 

 body. His observations on the germ-cells of the horse lead him to accept 

 the opinion of Regaud (1909, 1910) that "the appearance or failure of 



