BIGELOW: EAELY DEVELOPMENT OF LEPAS. 71 



found in contact, as shown in Plate 2, Figure 20, which suggests that 

 there is retardation in the approach of the pronuclei in cases similar to 

 Figure 19. All my observations point to the conclusion that the pro- 

 nuclei usually come into contact during the time when the yolk-lobe is 

 disappearing, and the egg is assuming the ellipsoidal form, that is, in 

 stages corresponding to Figures 4-6. 



Review of Literature on Maturation and Fertilization. 



A general review of the literature on these phases of cirripede devel- 

 opment is given by Groom ('94), consequently reference will not be made 

 in this connection to writings unless they have direct bearing upon 

 observations recorded in this paper. 



The formation of polar bodies and vitelline membrane have been ob- 

 served and described by Weisrnann und Ischikawa ('88), Xussbaum ('89), 

 Solger ('90), Groom ('94), and others. My observations on the forma- 

 tion of these structures are merely confirmatory of these earlier writers, 

 and have been recorded simply to complete my account of associated 

 phenomena. 



The contractions of the egg during elongation and the segregation of 

 protoplasm and yolk have been observed by Groom and others ; but the 

 process has, apparently, not been followed continuously, and has been 

 confused with the first cleavage, as will be shown in the review of litera- 

 ture bearing on that stage. 



Groom ('94, p. 133) states that in the unfertilized ovum of Lepas 

 anatifera no difference can be distinguished between the two poles, and 

 suggests that the ovum may become oriented only upon fertilization. 

 Opposed to such conclusion is the fact that in eggs taken from the ovi- 

 ducts the first maturation spindle marks the chief axis of the egg, which 

 thus seems to be determined long before fertilization. Nussbaurn ('90) 

 correctly observed that the axes of the embryo are established with the 

 formation of the polar bodies. 



Groom ('94, p. 136) states that "the axis of the spindle of the seg- 

 mentation-nucleus is not at right angles to that of the second directive 

 spindle." In the account of the first cleavage it will be shown that, in 

 opposition to this view, the first cleavage spindle is formed in a plane 

 perpendicular to the chief axis of the egg, with which the second matu- 

 ration spindle coincides at the moment when the polar cell is separated. 

 There is, therefore, in Lepas complete agreement with the usual condi- 

 tion in the eggs of other animals. 



