THE FORMATION OF THE MESODERM. 121 



regarding the origin of the mesoderm in Janthina. There are also various 

 other descriptions of the origin of the mesoderm in the Gastropoda which, an 

 they are still less well founded, cannot here be considered. 



The mesoderm arises in Patella, as above, in connection with the 

 entoderm, but at a stage when further differentiation has taken place 

 in the embryo (Patten, No. 83). In Patella, there is a blastula, 

 from which the entoderm arises/as already shown, by the ingrowth 

 of large cells at the vegetative pole (Figs. 49 and 50). There are at 

 first four large cells, occupying the same relative positions as do the 

 four macromeres in other forms. Now r , however, according to 

 Patten, a cell arises on each side of these four blastomeres which, 

 by division, gives oft* into the cleavage -cavity another rather large 

 cell. These two cells are regarded by Patten as mesentoderm-cells 

 (Fig. 50 A, em), and from them the two primitive mesoderm-cells 

 (mesodermal teloblasts) are derived. These lie near the blastopore, 

 at the posterior end of the larva and increase in number later from 

 behind forward (teloblastically). In this way the mesoderm-bands 

 arise, these being, according to Patten, developed with special 

 regularity in Patdla (Figs. 51 and 52, p. 126). 



The mesoderm, in the Gastropods, has generally been considered to arise in 

 connection with the primitive entomeres before the formation of the arch- 

 enteron, but it has recently been asserted that it arises in the form of coelomic 

 sacs, an assertion which was specially startling because it was supposed that 

 the Molluscs showed no sign of the formation of enterocoeles, such condi- 

 tions having so far never been observed. In the differentiation of the meso- 

 derm, especially in the development of the pericardium, the Mollusca, it is 

 true, shew great agreement with certain " Enterocoelia," and there is no doubt 

 that, like these, they possess a secondary body -cavity, but, in this respect, they 

 approximate most nearly towards the Annelida, the formation of the meso- 

 derm from mesodermal teloblasts being like that in the latter group. Con- 

 sidering all that is as yet known of the formation of the mesoderm, we cannot 

 agree with the results obtained by Erlanger for Paludina, and must continue 

 to be sceptical about them until they are better supported or are actually 

 confirmed by new investigations (if possible made on other forms as well). 



v. Erlanger's account is as follows : From the rather wide archenteron of 

 Paludina a bilobed outgrowth appears which gives the impression of a double 

 coelomic sac such as occurs for instance in various Echinoderms (Vol. i., pp. 

 407-409). This sac, which rises from the archenteron near the blastopore, 

 becomes detached later from the entoderm and now represents a vesicle 

 closed on all sides and symmetrical in form. The outer and inner walls 

 approach the ectoderm and the entoderm respectively so that at this stage 

 we might speak of a somatic and a splanchnic layer. It is evident that, up 

 to this point, the condition of the mesoderm closely resembles that of the 

 coelomic sacs in other animals. This, however, soon changes, for the coelo- 

 mic sacs, by giving off single cells, break up altogether, leaving only two 



