No. I.] THE EMBRYOLOGY OF CREPIDULA. 39 



nucleus is slightly larger than the male, and this continues to 

 be true as long as the pronuclei can be recognized as such. 



Two polar bodies are shown in the figure at the upper pole, 

 of which the first formed is much the larger ; the chromatin in 

 this has already divided into two masses, though the cell body 

 is still undivided. 



At the vegetal pole of the egg there is frequently found a 

 rounded mass of hyaline substance, which stains homogene- 

 ously. It persists until after the first two cleavages, lying in 

 the furrow between the macromeres, but apparently attached 

 to one of them only. I am not satisfied as to the significance 

 of this body, but am inclined to believe that it is a remnant of 

 the stalk of attachment by which the ovum was fastened to 

 the basal membrane of the ovarian follicle. If this view be 

 correct, the polarity of the egg is determined in the ovary, the 

 vegetal pole lying next the membrane, the animal pole next 

 the lumen of the follicle. This is precisely the condition in 

 Unio (Lillie ('95), p. 10), where the point of attachment marks 

 the position of the micropyle. There is no micropyle in Cre- 

 pidula, and no need of any, since there is no o^gg membrane, 

 but this hyaline mass suggests the micropyle, not only because 

 it is located at the vegetal pole, and seems to be formed in the 

 same way, but still more remarkably, because the spermatozoan 

 usually, though not invariably, enters the egg at this spot. In 

 all cases the polarity of the &gg is definitely established long 

 before the polar bodies are formed, and if my interpretation of 

 the hyaline mass is correct, the animal and vegetal poles of the 

 ^gg are established at a very early stage in the ovary. ^ 



1 Since the above was written a brief study of the eggs of Fulgur carica and of 

 Sycotypus canaliculatus shows that a similar body, though very much larger than 

 that in Crepidula, is present in these animals. In both Fulgur and Sycotypus 

 this body contains a considerable amount of yolk and yet stains quite uniformly, 

 as it does in Crepidula. 



I am convinced that this peculiar body is homologous with the problematical 

 lobe which is described by Mead ('95) in the egg of Chaetopterus, and further, 

 it is probably identical with the polar rings observed by Whitman ('78) in Clep- 

 sine, and since then by various authors in different annelids. 



