MUSEUM OF COMPARATIVE ZOOLOGY. 49 



rapidity, for when the egg has attained a diameter of about 600 /x 

 (Plate VIII. Fig. 3, Plate IX. Figs. 4, 5) they may have reached the 

 length of 3-3.5 fx. In this stage the layer when seen from the surface 

 presents an appearance (lower half of Fig. 3, Plate VIII.) which so 

 closely resembles that of the zona radiata in the mature egg, that one is 

 involuntarily led to believe that it is the zona. Even the peculiar ar- 

 rangement of the markings in curved lines recalls the appearance of the 

 zona when seen in a similar position. Notwithstanding the striking 

 resemblance, there cannot be the slightest doubt that this layer is not 

 the zona radiata. In radial sections it is difficult to distinguish between 

 a layer composed of a homogeneous matrix pierced with minute parallel 

 canals, and one composed of parallel rod-like structures, but in surface 

 views this is much easier. Careful focusing shows the same optical 

 properties as were observed in the earlier stage, and with much greater 

 distinctness. The staining, too, is such as is to be observed in the villous 

 layer rather than in the zona ; for the highly refractive bodies take the 

 deeper stain, the intervening substance having the paler color of the yolk. 

 But the last possibility of doubt concerning the nature of this layer is dis- 

 pelled by the appearance presented when the elements which compose it 

 are separated from each other. It frequently happens in mounting thin 

 sections that portions of the layer are detached, and even resolved into 

 their constituent elements. In such cases clusters of two or three rod- 

 like bodies, and even single ones, can be found in such proximity to the 

 layer as to leave no doubt that they are elements detached from it. 

 They have the same length and thickness as the markings of the layer; 

 they are highly refractive and deeply stained. They can in no way 

 correspond to anything that is observed in the zona radiata, but do 

 resemble in several particulars the villi of older eggs. 



From all this evidence I am certain that the layer which is first to 

 make its appearance between the yolk and the follicular epithelium is the 

 villous layer. 



In this stage, too, the union of the layer with the yolk is much more 

 intimate than its relation to the granulosa. The latter is often sepa- 

 rated from the layer, the yolk never. 



The cells of the follicular epithelium (Plate VIII. Fig. 3) have be- 

 come somewhat smaller than in the previous stage, but their nuclei 

 retain the same dimensions and the same lobed appearance which they 

 had during the earlier stage. As a consequence, the nuclei are closer 

 together. It will be seen that in the stage figured on Plate VIII. 

 Fig. 3, the diameter of a single average-sized granulosa cell corresponds 



vol. xix. — no. 1. 4 



