THE DEVELOPMENT OF APLYSIA PUNCTATA. 

 Text-fig. 7.' 



505 



Diat^rammatic oi^tical sagittal section, seen from the vegetative 

 pole of an egg in the 17U-cell stage. The derivatives m, m', 2m, 

 •2m\ of the mesoteloblasts 31. 3P, are seen spreading in an 

 anterior direction from the region of the anal cells Ac. The 

 macromeres A and B have diverged from one another to form 

 a segmentation cavity. 



' Tliis and the preceding six text-fignres have been modified from 

 Carazzi's drawings. The notation of the various blastomeres throughout 

 the segmentation follows the system, now almost universally adopted 

 for the description of cell-lineages, of Wilson (" The Cell-lineage of 

 Nereis."" • Journ. Morph.," vi). slightly modified by Conklin (-'The 

 Embryology of Crepidula," ' Journ. Morph.,' xiii). A, B, C, D are the 

 macromeres from which the successive quartettes of micromeres are 

 divided off ; the quartettes being distinguished by the co-eflficients 1, 2. 3. 

 Thus the first quartette will consist of la-\d, and its derivatives 

 will be la\ Id', ld\ Id- ; while the descendants of the latter generation 

 will be la'-», la'--, la-', l«-'-ld'-', ld'-\ Id^', \d--\ By the division of 

 D M is formed, which, since it contains the material for the production 

 of the mesoderm, is designated by the letter M. This later divides to 

 form the mesoteloblast Mand M\ and from these, after the separation of 

 a mesentoblast from each, small cells m and m\ etc., are budded off 



