88 bulletin: museum of comparative zoology. 



tion. A spindle parallel with the chief axis would be in harmony with 

 Groom's view that the first cleavage furrow is perpendicular to tliat 

 axis. Numerous transparent preparations of entire eggs have convinced 

 nie that such is never the case. 



In the review of literature on maturation and fertilization I have 

 already referred to Groom's mistake in identifying the pronuclei as the 

 daughter-nuclei of the segmentation nucleus. He speaks (p. 145) of two 

 nuclei seen in "the first blastomere " (cell ah'^ of this paper). One of 

 the two nuclei which he regards as the daughter-nuclei of the segmenta- 

 tion nucleus remains as the nucleus of " the first blastomere," the other 

 j)as8e3 into the "yolk hemisphere" (yolk-cell cd? in this account) just 

 before the cell-plate is formed. This is certainly erroneous, and is ap- 

 parently the result of his interpretation of the transverse furrow accom- 

 panying maturation as tiie cleavage furrow. In Groom's Figure 8 two 

 distinct nuclei are represented in the "protoplasmic" part of the egg, 

 which he considered " the first blastomere." It is evident from my 

 fiirures that the dauerhter-nuclei of the segmentation nucleus could not 

 normally get into such a position ; but the pronuclei are often seen on 

 one side of the constriction during maturation phases (see my Figure 

 18). I interpret Groom's Figure 8 as representing the pre-cleavage 

 stage corresponding to my Figures 3 and 18, and the lower half of the 

 egg as the yolk-lobe, not the yolk-cell cd^. I have already stated that, 

 unless eggs are kept under continuous observation, it is easy to confuse 

 this stage with the two-cell stage, when only living eggs are examined. 

 My series of figures shows that no such interpi-etation as that above 

 quoted fits the facts. There are two nuclei (pronuclei) in the proto- 

 plasmic hemisphere during the later maturation phases (Figs. 18, 20) ; 

 but in the " first blastomere " (cell ah^ in my Figs. 26, 27) there are never 

 two, one of which is destined to pass into the yolk. Groom's description 

 of the " yolk " (cell cd}) as at first without a nucleus, but receiving one 

 from the " first formed blastomere " (first micromere ab'''), is erroneous. 

 Neither cell can be said to receive a nucleus from the other, fur the 

 division of the segmentation nucleus, and the formation of the first 

 cleavage plane is such as ordinarily takes place in unequal cell division. 



The last statement applies also to allthe later cleavages. The micro- 

 meres rich in protoplasm, which are later cut off from the yolk-macro- 

 mere, cannot be said to give rise to a nucleus which migrates into the 

 yolk before complete separation of the " protoplasmic " cell. 



