BIGELOW: EARLY DEVELOPMENT OF LEPAS. 117 



lar. Such differences show well the morphological insignificance of the 

 details of cell division in the present case, for the Nauplii vary pro- 

 portionately much less; every one of the numerous, simple, or com- 

 pound bristles or spines of the Nauplius has its definite character and 

 position, which are maintained with surprising constancy throughout, 

 although they must have been produced by epiblast cells having very 

 different modes of origin and arrangement." 



In the preceding account of the various stages of cleavage this sup- 

 posed great variation in development has been discussed. It has been 

 shown that the development is extremely regular, and that there is not 

 the slightest foundation for views such as those above quoted. 



In a preliminary paper on L. fascicularis (Bigelow, '96) the results 

 were summarized as follows : " In all important respects the cleavage 

 of L. fascicularis is as regular as is ordinarily found in other Metazoa. 

 All previous observers have failed to recognize any definite order in the 

 cleavage of cirripede ova. It has always been described as exceedingly 

 variable, irregular and sui generis. There is undoubtedly some irregu- 

 larity and variation in the cleavage of the ova of those cirripedes where 

 a great amount of yolk is present. However, as will be pointed out in 

 a future paper, the cleavage of these forms, when interpreted by the 

 cleavage of L. fascicularis, is seen to follow a much more regular order 

 than has been supposed." 



Later studies have completely supported this interpretation, and even 

 the irregularity of development which I formerly believed to exist in 

 the case of those cirripedes whose ova have much yolk, appears not to 

 exist in the course of normal development. More extended study has 

 shown that L. anatifera, one of the forms which I at first interpreted as 

 somewhat variable in its development, is extremely regular. Studies 

 now in progress on other genera support the conclusion which I have 

 drawn from L. fascicularis and L. anatifera, namely, that the evidence 

 derived from a study of cell-lineage indicates that the development 

 of Lepas is as regular as the well known cases among gasteropods and 

 annelids. 



13. NOTES ON CLEAVAGE AND GERM-LAYERS IN L. FASCICULARIS. 



The early development of Lepas fascicularis is so closely like that 

 already described in the case of L. anatifera that extensive special 

 description is unnecessary, but some remarks are needed in order to 

 correct and supplement a preliminary note on this species which I 

 published in 1896* 



