70 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



cleavage). They beloug more properly to the maturation j)hases, and 

 have many characteristics known for ova of other groups of animals. 

 The polar axis thus established in the cirripede ovum has the same rela- 

 tion to polar cells, maturation spindles, and first segmentation spindle, 

 as is found ordinarily in telolecithal ova. 



The phenomena occurring during the elongation and distribution of 

 the materials of the cirripede egg, especially the formation of a constric- 

 tion which marks off a yolk-lobe at the vegetative pole, are apparently 

 similar to conditions which obtain in some molluscan eggs ; for example, 

 in the gasteropods Nassa (Bobrctzky, '76) and Ilyanassa (Crampton, 

 '96). In these cases the formation of the yolk-lobe closely resembles 

 that process in Lepas, but its later history is widely different. At one 

 stage of the maturation, the eggs of Nassa and Ilyanassa have a form 

 similar to that of the egg of Lepas as represented in Figure 3, a 

 constriction marking off a yolk-lobe. Whereas in the cirripede the con- 

 striction disappears before the first cleavage, in the gasteropods the first 

 cleavage plane forms so that in the unequal division a smaller cell (ah) 

 is separated from a larger one (cc?), which still retains the yolk-lobe. 

 After cleavage the yolk-lobe gradually disappears and the cell cd becomes 

 spheroidal in form. In Lepas, as in Nassa and Ilyanassa, the materials 

 composing the yolk-lobe are after the first cleavage contained in the cell 

 cd. 



In my attempts to determine the precise time of penetration of the 

 spermatozoon I have failed, as have all earlier investigators ; but we may 

 infer that it enters before the formation of the vitelline membrane, 

 probably about the time when the first polar cell is separated. In sec- 

 tions similar to that represented in Plate 2, Figure 17 (formation of 

 second polar coll) I have noted a darkly staining body near the vegeta- 

 tive pole of the egg. I am not certain of having identified the male 

 pronucleus in a stage earlier than one corresponding in external form to 

 Figures 3 and 18, in which, however, the pronuclei were widely separated, 

 as shown in Figure 19. A further comparison of Figures 18 and 19 

 shows that there is not a constant relation between the relative posi- 

 tions of the pronuclei and the telolecithal .distribution of the yolk and 

 protoplasm. In external outline and in the, presence of tlie constriction 

 marking off the yolk-lobe, the egg represented in Figure 18, correspond- 

 ing to Figure 3, is earlier than that shown in Figure 19, which cor- 

 responds to Figure G. But in Figure 18 the size and contact of the 

 pronuclei indicate an older stage than that of Figure 19. 



After the disappearance of the yolk-lobe the pronuclei are usually 



