THE FORMATION OK THE MESODERM. 



119 



macromeres described in the last-named form seems to represent the 



method of formation of the mesoderm most commonly found in the 



Gastropoda.* 



Far greater modification seems to prevail in the formation of the 



mesoderm in those forms which, like Nassa, are exceedingly rich in 



yolk, and yet it appears to its that it would be possible to trace back 



* [The early developmental history of the mesoderm has now been investi- 

 gated in so many different Gastropods, all of which show such close agree- 

 ment on this point, that we must carefully bear in mind the possibility of this 

 method of mesodenn-formation being typical of the entire group. The -meso- 

 derm almost invariably first appears as a single cell which is constricted 

 from one of the posterior macromeres ; this unpaired mesomere then 

 divides into two cells, bilaterally arranged, which, as mesodermal teloblasts, 

 give origin to the paired mesoderm-bands. The macromere from which 

 the first mesomere originates is possibly the left posterior in all dextral 

 (ni-~tropods, and the right posterior in sinistral forms (Crampton). In the 

 great majority of the Gastropoda, soon after the last quartette of micromeres 

 has arisen, this macromere divides, thus giving origin to two cells, one of 

 which is an entomere, while the other is usually the primary mesomere, more 

 rarelv, Patella Patten), and Crepidula (Conklin now withdraws the account 

 given above), the two cells represent an entomere and a mesentomere, the 

 complete separation of the mesoderm from the entoderm only taking place 

 after further divisions. The origin of the inesoderm in Crepidula is expressed 

 by Coxklim as follows : — 



'm 1 small mesomere. 



fME 



D 



(left posterior 

 macromere). 



ME 1 (right)-; 



Me 1 



I , p [ fM 1 mesodermal teloblast. 

 1 e {e 1 secondary entomere. 



E 1 primary entomere. 



E- primary entomere. 



V- '- ' e " secondary entomere. 

 . ., j e \ M 2 mesodermal teloblast. 

 L ME 2 (left) l ' 



I m- small mesomere. 



D ento- 

 mere. 



In the majority of Gastropoda in which this point has been investigated, 

 as, for instance, Planorbis, Limax, Physa, Siphonaria, Tethys, Umbrella, etc., 

 the condition is, as stated above, much simpler and may be expressed thus : — 



MM 



1>- 



( M 1 right mesodermal teloblast. 

 ( M 2 left mesodermal teloblast. 



D entomere. 



While the greater part of the mesoderm arises from the paired mesoderm- 

 bands, a smaller and more scattered portion appears to arise on either side of 

 the body from the ectoderm. This was suggested by Heymons in Umbrella 

 (No. XII.) and has since been confirmed by Conklin for Crepidula (No. IV.) 

 and Wierzejski for Physa (No. XXVII.) ; the scattered mesoderm has been 

 compared with the larval mesoderm of Unio (Lilue). — Ed.] 



