430 



TREADWELL. 



[Vol. XVII. 



losing their connection with the wall of the archenteron. The 

 two small cells described as budded posteriorly from 4di and 4d2 

 form a part of the wall of the archenteron. Since the greater 



part of 4d gives rise to the 

 definitive mesoblast, it is 

 therefore, to use Conklin's 

 term, a mesentoblast cell. 

 The cells 4di.i and 4d2.i 

 at first lie in the wall of 

 the archenteron, though 

 they protrude considerably 

 into the segmentation cav- 

 ity. Consequently, in a 

 section passing a little to 

 one side of the sagittal 

 plane, they seem to lie 



Xti ME 



Fig. 5. — Beginning of entodermal invagination. ^,^, , ■,, • i • ., ,-n< 



entoderm; Im, larval mesoblast; ME, 4d before Wholly m thlS CaVlty ( 1 CXt- 



the final separation of mesoderm from entoderm ; .£■, "pjo- ^\ Fach buds off a 

 the first entoderm cell budded off from i^z- ~^\-i> 



the " anal " cell. sccond Small ccll (PI. XL, 



Fig. 56), and the larger 

 portion is the definitive 

 mesoblast, Mj and Mg. 

 Considerably later (Fig. 

 57, PI. XL, and Text-Fig, 

 8) each divides equally. 

 Only until this last divi- 

 sion do these cells entirely 

 lose their connection with 

 the archenteric wall. Text- 

 Fig. 8 is an optical section 

 of the specimen drawn in 

 Fig. 57, and shows that 

 the descendants of 4d are 

 still connected with the 

 entoderm. I have actual 

 sections of this stage which show exactly the same thing. The 

 products of the last division migrate rapidly through the 

 segmentation cavity, retaining approximately their original 



Fig. 6. — Later stage of invagination. 

 a, archenteric cavity. 



