188 GASTEROPODA AND PTEROPODA. 



the blastopore, which then gradually make their way dorsalwards and 

 forwards, and form a complete layer between the epiblast and 

 hypoblast. The above general mode of formation of the mesoblast 

 may be seen in fig. 107, representing three stages in the develop- 

 ment of Paludina. 



In some cases the mesoblast arises from certain of the sesrmenta- 



o 



tion spheres intermediate in size between the epiblast and hypoblast 

 spheres. This is the case in Nassa mutabilis, where the mesoblast 

 appears when the epiblast only forms a very small cap at the forma- 

 tive pole of the ovum ; and in this case the mesoblast cells accompany 

 the epiblast cells in their growth over the hypoblast (fig. 105). 



In other cases the exact derivation of the mesoblast cells is quite 

 uncertain. The evidence is perhaps in favour of their originating 

 from the hypoblast. It is also uncertain whether the mesoblast is 

 bilaterally symmetrical at the time of its origin. It is stated by 

 Rabl to be so in Lymnseus 1 . 



In the case of Paludina the mesoblast becomes two layers thick, 

 and then splits into a splanchnic and somatic layer, of which the 

 former attaches itself to the hypoblast, and gives rise to the muscular 

 and connective-tissue wall of the alimentary tract, and the latter 

 attaches itself to the epiblast, and forms the muscular and connective- 

 tissue wall of the body and other structures. The two layers remain 

 connected by protoplasmic strands, and the space between them 

 forms the bod} 7 cavity (fig. 107). In most instances there would 

 appear to be at first no such definite splitting of the mesoblast, but 

 the layer has the form of a scattered network of cells between the 

 epiblast and the hypoblast. Finally certain of the cells form a 

 definite layer over the walls of the alimentary canal, and constitute 

 the splanchnic mesoblast, and the remaining cells constitute the 

 somatic mesoblast. 



We must now return to the embryo at the time when the blastopore 

 is becoming narrowed. First of all it will be necessary to define the 

 terms to be applied to the various regions of the body and these 

 will best be understood by taking a fully formed larva such as that 

 represented in fig. 101. The ventral surface I consider to be that 

 comprised between the mouth (ni) and the anus, which is very nearly 



1 Eabl (No. 268) has quite recently given a more detailed account than previous 

 observers of the origin of the mesoblast in Plauorbis. He finds that it originates 

 from the posterior one of the four large cells which remain distinct throughout the 

 segmentation. By the division of this cell two 'mesoblasts' are formed, one on each 

 side of the middle line at the hinder end of the embryo. Each of these again divides 

 into two, an anterior and a posterior. By the division of the mesoblasts there arise 

 two linear rows of mesoblastic cells the mesoblastic bands which are directed forwards 

 and divided transversely into two parts, an anterior continued from the front mesoblast, 

 and a posterior from the hinder mesoblast. 



If Rabl's account is correct, there is a striking similarity between the origin of the 

 mesoblast in Mollusca and in Chastopoda. It appears to me very probable that the 

 mesoblastic bauds are formed (as in Lumbricus) not only from the products of the 

 division of the mesoblasts, but also from cells budded off from one or both of the 

 primary germinal layers. 



