122 



GASTROPODA. 



insignificant vesicular vestiges surrounded by irregularly distributed meso- 

 derm-cells on the ventral side of the archenteron. These will be referred to 

 again later.* 



Other descriptions in which the middle germ-layer is derived direct from 

 the ectoderm are difficult to reconcile with the accounts we have given of the 

 formation of the mesoderm. Such an ectodermal origin is attributed to the 

 mesoderm in Fusus (Bobretzky, No. 11) in Vermetus (Salensky, No. 99) and 

 in various other Gastropods (Fol). The eggs of Vermetus are very rich in yolk.. 

 The ectoderm lies as a thin layer upon the macromeres, almost entirely en- 

 closing them. Near the blastopore, the increase in number of the cells of the 

 ectoderm is said to give rise to a thickening which is the rudiment of the 

 mesoderm. In Fusus, Bobretzky regards the latter as arising by a prolifera- 

 tion of cells from the lips of the blastopore. According to Salensky, this 

 mesoderm-rudiment is bilaterally symmetrical like the mesoderm-bands, but 

 another independent formation of mesoderm is said to take place in the 

 neighbourhood of the shell-gland. Salensky is inclined to regard this part 

 of the mesoderm as having arisen through delamination from the ectoderm 

 near which it lies, i.e., from the dorsal part of the body. There is some 

 similarity between this last view and the account given previously by P. 

 Sarasin (No. 101) of the origin of the mesoderm. According to Sarasin, 

 growths of the ectoderm occur at certain points of the body from which 

 mesodermal elements become detached. This takes place partly at an early 

 stage of embryonic development and partly later. Since this material becomes 

 abstricted at various times and at different parts for the formation of those 

 organs which are usually regarded as mesodermal, Sarasin is unable to assume 

 the existence of one uniform mesoderm-layer and therefore takes somewhat 

 the same stand-point as that adopted later by Kleinenberg in so decided a 

 manner for the Annelida (Vol. i., pp. 292 and 293). Primitive mesoderm- 

 cells and mesoderm-bands in Bythinia have been more recently described by 

 Erlanger (No. 28) and, according to the very definite account of Sarasin, we 

 should have to show whether, besides this distinct mesoderm-rudiment, a 

 further formation of mesodermal elements takes place from the ectoderm, as 



* [In spite of the more recent investigations on this point, the true origin of 

 the mesoderm in Pabulum must still be regarded as undecided. In his 

 most recent publication, Erlanger (No. N.) gives figures which are difficult to 

 interpret in any other way than he has done. Consequently, he still regards 

 Paludina as enterocoelic, but he finds, besides the coelomic sac, paired 

 primitive mesoderm-cells near the blastopore which may be the forerunners 

 of the cells which form the enterocoeles. He suggests that the sparsity of 

 yolk has made Paludina more primitive in this respect than other Gastropoda. 

 Tonniges (No. XXV.). who has specially investigated this point in Paludina, 

 concludes, but without reference to Erlanger's latest work, that the meso- 

 derm arises shortly after the formation of the gastrula by a wandering in of 

 ectoderm-cells from that portion of the ventral surface which is formed by the 

 closing of the blastopore ; the mesoderm then spreads out to form a ventral 

 sheet which extends by growth on either side of the archenteron. Soon, how- 

 ever, its cells become scattered in the cleavage-cavity without forming a second- 

 ary coelom. Schmidt (Nos. XX. and XXL), who has confined his attention to 

 Pulmonates, finds no support for Erlanger's views in the origin of the meso- 

 derm of these forms. An investigation on this point in some of the primitive 

 Prosobranchia is very desirable. — Ed.] 



