18 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



than the thickness of the egg-membrane (Fig. 3). The explanation of 

 this would seem to be, that the micropyle is pulled inward during its con- 

 tact with the egg, producing a funnel-like depression in the membrane, 

 which is obliterated the moment the connection is broken. This would 

 account, in part at least, for the projection of the polar globule. We have 

 (hen a micropylar depression to distinguish from the proper micropylar 

 funnel. It is only the depression that is obliterated. The funnel itself 

 persists, and we have found it in late embryonic stages. Its position with 

 relation to the germinal pole is liable to change, and its disappearance in 

 this way has led some writers to suppose that its obliteration is complete. 

 It follows from these observations, that the escape of the polar globules 

 through the micropyle is not a normal occurrence ; and the idea that they 

 are regularly lodged in the micropyle and serve as a mechanical means 

 of excluding spermatozoa is wholly untenable. Calberla 9 suggested that 

 the tail of the spermatozoon is left in the micropylar canal for the purpose 

 of blocking the way to other spermatozoa; and Fol 10 claimed that polysper- 

 matic fecundation is prevented in the echinoderm egg by the rapid forma- 

 tion of an impenetrable vitelline membrane at the moment when the 

 spermatozoiin comes in contact with the vitellus. The objections to these 

 views have already been reviewed by one of us, 11 and the conclusion 

 reached that the receptivity of the ovum is regulated by physiological 

 conditions, not by mechanical contrivances. 



B. History of the Pronuclei. 



Plates XXII., XXIII., and XX. Fig. 4 a-c. 



1. Appearance in the Living Egg. 



In these transparent eggs one might expect to be able to see the nuclear 

 bodies, and to follow their movements. But under a magnifying power of 

 about 300 diameters — the highest that could be employed in such work — 

 the pronuclei remained invisible until just before conjugation began. At 

 this time — about 20-23 minutes after fertilization— they appear as two 

 equal vesicles, 10 fi in diameter, one above the other, and in contact or 

 nearly so. At the moment of contact they are perfectly spherical. Soon 



9 G. E. Calberla. Dor Befruclitungsvorgang beim Ei von Petromyzon Plancri. Zeitschr. f. w. 

 Zoologie, XXX., 1877. 



10 Hermann Fol. Reelierclies sur la Eecondation et le Commencement de l'Henogenic, 1879. 



11 C. O. Whitman. Ookinesis, Journ. Morpli., 1. ••>. IS87 



