92 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the blastoderm, contains only ectoblastic material. An exception is to 

 be noted in the case of Nussbaum, who saw the mesoblast apparently 

 proliferating from the edge of the blastoderm. The cell ab 2 according 

 to his interpretation, then, contains all the ectoblast and the mesoblast. 



The erroneous interpretations of the earlier observers are largely 

 explained by the fact that their observations were almost exclusively 

 confined to living eggs, in which the nuclear conditions are hidden. 

 Without sections or transparent preparations divisions of the yolk-cell 

 might be easily overlooked. Lang ('78) and Nassonow ('87) figured for 

 Balanus, and Nussbaum ('90) for Pollicipes, distinct protoplasmic radi- 

 ations in the yolk-cell, but failed to see their significance as indicating 

 division. I am convinced that the structures seen were asters or archo- 

 plasmic radiations. Korschelt nnd Heider ('90) made the suggestion, 

 based on Nassonow's figures, that the yolk-cell cd z divides and contributes 

 cells to the blastoderm. 



Groom ('94) described the yolk-cell cd 2 in the case of all cirripedes 

 whose development he observed, as a macromere giving rise in succession 

 to a number of " blastomeres," which are added to the blastoderm. He 

 proved conclusively that the "protoplasmic" cell ab 2 (his "first blasto- 

 mere," my " first micromere ") does not give rise to all of the ectoblast, 

 as supposed by all previous observers. According to his account several 

 cells (estimated at nine or ten) are cut off from the yolk-cell after the 

 first cleavage, and with the derivatives of the " first blastomere " form 

 the blastoderm. 



Several years ago, without knowledge of Groom's results, owing to the 

 inaccessibility of the literature, I ('96) found that in Lepas fascicularis 

 the yolk-macromere divides several times, practically synchronously with 

 the divisions of the other cells, thus contributing to the formation of 

 stages of 2, 4, 8, 16 and 32 cells. This confirmed Groom's results in 

 general ; but as to the order, method, and number of the divisions I was 

 forced to dissent from his account. 



According to Groom's description there is great variation in the num- 

 ber, order, and position of cleavages both in the yolk-cell and in the 

 other cells of the cleaving egg. He concluded that the cleavage of the 

 cirripede egg is decidedly irregular. He writes (p. 140), "there is no 

 constancy in the mode of growth of the blastoderm over the yolk ; " and 

 mentions (pp. 139-140) many of the variations which occur. 



Many of these supposed variations are certainly misinterpretations 

 due to errors in orientation, and others are apparently based upon ab- 

 normal eggs. Mention may be made of several cases. Groom states 



