THE FORMATION OF THE MESODERM. 



117 



BROOKS' description, the entoderm forms in an exactly similar way 

 in Urosalpinx. A mass of food -yolk is also formed in Fusus, Ver- 

 metms, Aplysia, etc., by those macromeres which attain to so con- 

 siderable a size. 



The Mesoderm. In connection with the account given of the 

 processes of cleavage, it was stated that the middle germ -layer arises 

 very early. In Planarbis, one of the posterior of the four macro- 



FIG. 47. A-D, longitudinal sections through embryos of different ages of Nassa muta- 

 bilis (after BOBRBTZKY, from BALFOUR'S Text-book), bl, blastopore ; ep, ectoderm ; 

 /, rudiment of the foot ; hy, entoderm ; in, intestine ; ra, mouth ; me, mesoderm ; 

 sg, shell -gland ; st, enteron. 



meres * divides, giving rise to an entomere and to the primitive meso- 

 mere, which latter eventually yields the two primitive mesoderm- 

 cells, as already shown (p. 110). These are soon pressed into the 

 cleavage-cavity, and, by their increase in number, give rise to the 

 two mesoderm-bands. This seems also to be the case in the Ptero- 



* [This cell is believed to be homologous in all Gastropods and is now desig- 

 nated D by students of cell-lineage. ED.] 



