388 E. RAY LANKE3TER. 



in some cases, when the gastrula's ectoderm and stomach- 

 wall are differentiated by invagination, a certain number of 

 the primitive segmentation-cells should remain involved, 

 neither in the one nor the other, but lying intermediately, 

 thus forming a simultaneously differentiated mesoblast, but 

 even then we should be able to trace such cells to an earlier 

 connection with the cells of either epiblast or hypoblast. In 

 point of fact, such cases have not been brought forward from 

 actual observation for discussion. 



Thus, then, the term mesoblast and its correlative term " tri- 

 ploblastic" have not reference to the existence of an embryonic 

 layer of co-ordinate value with the two primary layers, but 

 to a disposition and growth of some of the early structural 

 elements of the higher animals in and around a strongly 

 marked space separating the two primary layers. 



Summary. — The observations of fact which have been 

 brought forward above are to a large extent disjointed, and 

 even as far as concerns the period of development to which 

 they refer, very far from exhaustive. They must rather be 

 regarded as suggesting the desirability of more detailed and 

 long-continued study. 



As evidence of the value which may be assigned to them, 

 I shall quote the summary of the development of the water- 

 Pulmonata, given by Keferstein in Bronn's invaluable 

 * Thierreich,' followed by a statement of the points in which 

 my observations traverse or supply important omissions in 

 that summary. 



Keferstein says (p. 1230 of the third volume of the above- 

 named work) : 



" The Pulmonata of fresh waters exhibit the closest agree- 

 ment with the Prosobranchiata, excepting that in them all 

 trace of a velum is wanting (a). We possess very numerous 

 and elaborate memoirs on the development of Limnceus, espe- 

 cially by Stiebel, Dumortier, Pouchet, Karsch, Warneck, 

 Lereboullet, &c. ; on that of Pla?iorbis we have the researches 

 of Jacquemin, &c. ; so that the facts are here accurately 

 known. We confine ourselves in the following remarks to 

 Limnaus. 



" Two hours after the egg is laid its cleavage commences, 

 as a consequence of which at once one or two so-called 

 Richtungsblaschen are pushed out by the contraction of the 

 yelk, and then a first circumferential cleft extends itself 

 round the spherical egg-mass. The germinal vesicle is no 

 longer visible in the impregnated yelk ; but shortly before the 

 equatorial cleft is formed a clear speck is visible in the yelk, 

 which, according to Warneck, becomes biscuit-shaped, and 



