Volume /.] May, 1900. \_No. j. 



BIOLOGICAL BULLETIN. 



THE EARLY CLEAVAGE AND FORMATION OF 



THE MESODERM OF SERPULORBIS 



SOUAMIGERUS CARPENTER. 



S. J. HOLMES. 



THE material upon which the present paper is based was 

 collected at San Pedro, Cal., in the summer of 1895. Work 

 upon it was carried on for a time during the winter of 1895-96, 

 under Prof. C. O. Whitman, at the University of Chicago ; but 

 as the series of stages the material afforded proved incomplete, 

 the subject was laid aside in the hope that, at some future time, 

 when new material could be collected, the gaps might be filled. 

 Since it is improbable that an opportunity of remedying this 

 defect will soon present itself, and as the development of this 

 form shows several interesting points of comparison as regards 

 the formation of the mesoderm with what has recently been 

 found to obtain in other mollusks and certain annelids, it was 

 thought best to publish the present account. The development 

 of Vermetus, a genus from which Serpulorbis is somewhat 

 doubtfully distinct, has been studied by Salensky J in consider- 

 able detail. According to Salensky, mesoblastic pole cells do 

 not appear, and the mesoderm in Vermetus arises at a compara- 

 tively late period of development by a proliferation from the 

 ectoderm in the region of the blastopore. With this conclu- 

 sion my own observations do not agree, as certain stages that 

 were found afford very clear evidence that the mesoderm arises 



1 Arc /lives de Biologic. I, vi, 1887. 

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