SEX-CORDS AND SPERMATOGONIA IN CHICK 405 



stitial tissue as compared witli tlie true connective tissue and 

 the seminiferous cords. I do not know whether there is an 

 actual decrease, or only apparent, due to the great increase in 

 the size of the testis. In any event I was not able to find any 

 signs of degeneration in the interstitial cells. 



It is an interesting fact that the first appearance of the inter- 

 stitial cells in the testis of the 13 day embryo is synchronous 

 with the appearance of the spermatogonia, and that they only 

 reach their maximum development when all the primordial 

 germ-cells have become spermatogonia. 



There is no doubt, in the chick, that they arise from the 

 connective tissue of the testis, which in its turn is derived from 

 the general mesenchyme of the mesonephros. That they arise 

 from the connective tissue elements by simple differentiation is 

 proven, first, by the presence of transitional forms at the time 

 of their origin, and secondly, by the very rapid increase in the 

 numbers of the interstitial cells in the absence of any evidence 

 of division. 



In a previous paper (Swift, '15), and in a preceding page of 

 this article, a short account was given of the arrangement of the 

 mitochondria around the attraction-sphere of the oogonia and 

 spermatogonia respectively. I shall hereafter call this body, 

 made up of sphere and mitochondria, the mitochondrial crescent. 



As has been previously stated, the mitochondrial crescent 

 consists of attraction-sphere and mitochondria, and occupies 

 that part of the cell in which the cytoplasm is most voluminous ; 

 in other words, it occupies a part of the vegetative pole of the 

 cell. The whole body in one plane, appears like a crescent 

 fixed on, or capping the nucleus, the attraction-sphere in the 

 middle enclosing the centrosomes, around it a thin clear area, 

 and extending down on either side the arms of the crescent 

 composed of mitochondria (figs. 3, 4, 5 and 6). In case the 

 knife does not section the nucleus, but passes through the sphere, 

 then the mitochondrial crescent appears as a circular mass, 

 composed of attraction-sphere, clear area and surrounding circle 

 of mitochondria (fig. 4). In nearly all cases the cytoplasm which 

 suspends the mitochondrial portion of the mitochondrial cres- 



