316 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



else an invagination-gastrula appears as a stage following the epibolic 

 gastrula, as, for instance, in Ostrea and various Gastropods, in which 

 ;a cleavage-cavity arises only at a later stage, and the macromeres 

 which are continuing to multiply press in towards it. There is also 

 no essential difference between these two types of formation of the 

 germ-layers. In the Cephalopoda also the germ-layers may be traced 

 back to the same processes, although these are influenced in a marked 

 manner by the large amount of yolk in the egg. 



The middle germ-layer arises in a very similar way in all those 

 forms in which it has been investigated'. It originates in two 

 primitive mesoderm-cells derived from one of the macromeres (primary 

 entoderm). In the Gastropoda, in which this point has been best 

 ^examined, the formation of the primitive mesoderm-cells from the 

 macromeres was found to be very regular.* The large primary cells 

 of the mesoderm have been discovered to exist in all the following 

 divisions of the Mollusca : the Amphineura, the Lamellibranchia, the 

 Solenoconchae and the Gastropoda. In the Cephalopoda, on the 

 contrary, the development of the mesoderm has been considerably 

 modified by the conditions mentioned above. 



The two mesoderm-bands arise through the multiplication of the 

 primitive mesoderm-cells. It has repeatedly been stated that 

 niesodermal tissue is not due to the multiplication of these cells, but 

 is yielded later partly by the ectoderm, as described for the Annelida. 

 'The first of these views, i.e., the derivation of the mesoderm from the 

 primary cells is, so far, the more probable, but the other view should 

 not be summarily dismissed, and in any case deserves more careful 

 investigation.f 



It is very characteristic of the Mollusca that the mesoderm-bands 

 .are retained for only a short time. They soon disintegrate, single 

 cells separating from them and becoming distributed in the cleavage- 

 cavity as the so-called mesenchyme. Before this happens, however, 

 or else during this process, a cavity appears in each of the mesoderm- 

 bands ; this is bounded by a more or less regular epithelial wall and 

 is thus recognisable as the coelom. This process is the same already 

 met with in the Annelida (Vol. i., p. 290), and Arthropoda (Vol. iii., 

 p. 413). In these latter it leads to the formation of two mesodermal 

 layers, one applied to the ectoderm and the other applied to the 

 entoderm ; these are the somatic and splanchnic layers. This seems 



[See footnote, p. 119, "ED.] . t[See p. 29, ED.] 



