BIGELOW : EARLY DEVELOPMENT OF LEPAS. 93 



that the " second blastomere " (cell c 8 , second micromere, in my figures) 

 may be formed on either side of the yolk-cell d 3 , and illustrates such 

 conditions by his Figures 10 and 12 (L. auatifera). There is nothing 

 in either his text or figures to prove that these are not entirely similar 

 eggs viewed from almost opposite poles. They were certainly drawn 

 from different points of view, and the apparently different positions 

 occupied by the " second blastomere " are thus easily explained. Like- 

 wise, the "third blastomere" (rf 4 * 2 , third micromere, in this paper) is 

 said to arise on either the right or left of the second. Groom's Figures 

 15 and 16 (L. anatifera), which illustrate this, are certainly views of two 

 similar eggs, and apparently the cell considered the " second blastomere " 

 is not the same in both cases. The position of the "third blastomere " 

 shown as " emerging from the yolk," in one figure on the right and in 

 the other on the left, I interpret as being near the animal pole of the egg. 

 A number of other cases of such results based upon uncertain orientation 

 of the egg might be drawn from Groom's paper ; but enough has been 

 said to show that his evidence is far from convincing, that there is much 

 variation even in the earliest stages, and that the assumed variability of 

 the later stages rests upon a very uncertain basis. In opposition to this 

 view of the cleavage of the cirripede egg as variable and irregular, I shall 

 give evidence supporting my interpretation of the cleavage of Lepas as 

 normally regular and constant. 



In this connection I wish to consider Groom's account of the method 

 in which the yolk-cell divides. The discussion will apply to the second 

 or any later cleavage by which blastoderm cells are cut off from the 

 yolk-cell, for the method of division is the same in all. 



The following quotations from Groom's paper give his interpretation 

 of the method by which new cells are formed from the yolk-cell. On 

 page 197 he writes : " As the first blastomere becomes cut off from the 

 yolk the nucleus divides and one daughter-nucleus passes into the yolk 

 half, and soon emerges accompanied by protoplasm to form a second 

 blastomere and generally situated close to the first. As this becomes 

 cut off from the yolk it gives off into the yolk a nucleus, which behav- 

 ing similarly to the daughter-nucleus of the germinal vesicle, forms new 

 protoplasm and emerges as a third blastomere. At each successive 

 stage the yolk is in communication with one merocyte or newly-forming 

 blastomere, and this, before becoming shut off as a blastomere, gives off 

 a single nucleus into the yolk." A similar statement on page 145 of 

 Groom's paper contains some other points to which it will be necessary 

 to refer. One daughter-nucleus of the segmentation nucleus is said to 



