496 BULLETIN OF THE 



come possessed of all the available nuclear fluid of the yolk ; that in the 

 former they imbibe this fluid to the same extent. These two experi- 

 mental cases he maintains will serve to explain diff'erences which exist in 

 different groups of animals. In these cases both of which are considered 

 normal, the yolk promptly retracts from the vitelline membrane after 

 impregnation, and one often sees a minute bridge of protoplasm connect- 

 ing the surftice of the yolk with the membrane near the vegetative pole, 

 — probably the place of penetration of the spermatozoon. If the impreg- 

 nation is undertaken either before or after the epochs mentioned, ab- 

 normal phenomena are the result. The yolk withdraws from the vitelline 

 membrane not at all, or only slowly. A number of isolated stellar figures 

 appear in the cortical portion of the yolk, and remain limited in extent. 

 If the fertilization is effected during the metamorphosis of the germina- 

 tive vesicle, the development proceeds normally to the end of the forma- 

 tion of the polar globules, but not further ; if it is effected too late (six 

 hours after exclusion) two or three of the male asters may approach 

 close to the egg nucleus, and the latter sometimes takes an oval form. 

 Normal segmentation does not follow in either of these cases. 



Hertwig disagrees with Fol as to the formation of a vitelline mem- 

 brane at the time the spermatozoon penetrates the egg. The vitelline 

 membrane, he claims, already existed, and the interval between it and 

 the yolk is brought about by the contraction of the latter, which is 

 accompanied by the pressing out of the perivitelline liquid already ob- 

 served by the older naturalists. 



The results which Calberla ('78) has reached in his recent paper on 

 the fecundation of the eggs of Petromyzon Planeri he has himself con- 

 densed into the following form (p. 477). A single spermatozoon enters 

 through the outer micropyle into the space between the egg membrane 

 and the yolk. This space is filled with protoplasm free from yolk gran- 

 ules. The contact of the spermatozoon with this sets in activity a 

 stimulus which results in a slight contraction — an amoeboid motion — 

 of the yolk, which makes itself apparent in a separation of this clear 

 layer from the egg membrane in the vicinity of the micropyle. This 

 partial separation of the egg membrane from the yolk now makes pos- 

 sible an influx of water into the perivitelline space thus formed. Such 

 an inflowing of water was previously prevented by the pores of the egg 

 membrane being sealed up by the peripheral layer of clear protoplasm. 

 By this influx of water the egg membrane is widely separated from the 

 yolk. Authors who ascribe the existence of a great space between the 

 vitellus and membrane in lower vertebrates to an extensive contraction 



