1877.] 167 . [Minot. 



certain Nematods, and even in so aberrant a form as Cucullanus 

 according to Biitschli, as likewise among the Ascidians and Amphi- 

 oxus (Kowalewski) and Bryozoa (Schmidt). Development by invag- 

 ination occurs also among the Coelenterates, but no accurate figures 

 or descriptions have ever been published of those stages in which the 

 difference in size of the cells ought to be seen, so that we cannot say 

 whether this division of animals conform, or are exceptions, to the 

 general rule. Among the sponges the distinction between the two 

 parts of the vesicular embryo is well marked (cf. the recent publica- 

 tions of F. E. Schulze, Barrois, Mecznikow, Carter, Keller, Schmidt 

 and Hyatt). It is very decided in Halisarca, but most striking in 

 the calcareous sponges, the embryos of which are nearly spherical, 

 one hemisphere consisting of small cells without large granules, the 

 other of very large cells containing a great deal of lecithe (dento- 

 plasm of Eld. van Beneden). The same peculiarities have been 

 described also in Planarians, whose eggs exhibit the finely granular 

 fast growing ectoderm at one pole (Girard, Keferstein). 



In all these cases a vesicle is formed by a single layer of cells, of 

 which there are two kinds: around one pole small ectodermic cells 

 growing out of a finely granulate protoplasm ; around the opposite 

 pole larger cells, which segment more slowly and frequently contain 

 a large amount of nutritive matter (Nahrungsdotter, lecithe, dento- 

 plasm). This mode of development may be distinguished as polar, a 

 term including epibolie, embolie, and all forms of invagination. 



There is a second mode of segmentation, generally known as de- 

 lamination, which leads to the formation of double walled vesicles. 

 Fol has studied the process of delamination in the Geryonidse, the 

 only instance in which the changes occurring during this mode of 

 development have been followed in detail; nevertheless it can al- 

 ready be positively affirmed that the same difference of growth em- 

 phasized above reoccurs here also. The ectoderm consists of small 

 cells, the enclosed entoderm (or yolk) of large cells, often with much 

 nutritive matter. Mr. Minot referred also to the observations of 

 Leuckart and others on Nematods and Platyhelminths, Villot on 

 Gordius, A. Agassiz, Kowalewski and Fol on Ctenophorae, fid. van 

 Beneden on Gammarus, Selenka on Phascolosoma, etc. 



In the case of Annelids and Vertebrates the development is ren- 

 dered much more complicated through the singular formation of the 

 body-axis. If we return for a moment to the Cephalopods, we find 

 that they are distinguished by the yolk (Nahrungsdotter) not seg- 



