878 SUMMARY OF CURRENT RESEARCHES RELATING TO 



as is so frequently the case with the Gastropoda, a holoblastic egg ; 

 the peculiar characters of its first four cells recall, however, to mind 

 the meroblastic arrangement, and on the whole it would seem to be an 

 interesting intermediate condition. 



After pointing out the striking resemblances exhibited by many 

 Gastropoda in the early history of their ectoderm, the author directs 

 attention to the fact that Hallez has described a very similar process 

 in the Turbellaria. The bilateral character of the development of the 

 mesoderm is insisted on, and its occurrence in allies is also noted. 

 The closure of the blastopore, and the later development of the mouth 

 at the same point, was observed. The gastrula itself is formed 

 by epiboly, in its essential characters, but on the other hand it pre- 

 sents some approach to the embolic method. It is true that the 

 ectoderm grows around the whole of the egg ; but it is just as true 

 that the endodermal cells are budded off in a manner which can only 

 be regarded as a modified process of invagination. The absence of 

 a cleavage-cavity is an epibolic feature ; and so it follows that Fusus, 

 Neritina, and Natica present a series of intermediate forms leading to 

 the well-marked emboly of Paludina ; and we have in the Proso- 

 branchiata an intermediate condition between the Pteropoda, whose 

 gastrula is always formed by epiboly, and the Pulmonata and Hetero- 

 poda, where the typical method is by emboly. 



The early appearance in Neritina of some organs, and especially 

 of the velum, is regarded as being analogous to the facts which have 

 been reported of the marine Prosobranchs ; the cells which become 

 grouped around the animal pole are possibly those which become the 

 velar cells. Both in Planorbis and in Neritina two kinds of cells can 

 be made out in the wall of the archenteron ; some are richly laden 

 with protolecitbin, while others are free from this body, and have the 

 appearance of mesodermal cells ; some similar characters have been 

 noted in the Pulmonata. 



Locomotor Organs of Cyclostoma elegans.* — In continuation of 

 his researches. Dr. H. Simroth deals here not only with the above 

 form, but gives a general account of his studies on all the German 

 Gastropoda. 



With regard to the former, we find that, as compared with Paludina, 

 it exhibits a marked concentration of the cells of the pedal nervous 

 system in the two pedal ganglia ; there are no peripheral commissures, 

 but between the ganglia there is a second commissure, the presence 

 of which can hardly be taken as an indication of anything but a 

 metameric arrangement. In his second part, the author shows that 

 the elongation of the foot is effected by longitudinal muscles ; in the 

 Prosobranchiata and Branchiopneusta the undulatory action is irre- 

 gularly distributed over the whole foot ; the pedal nerves have only one 

 branch in the foot. The Pulmonata are provided with a sympathetic 

 plexus, and the undulatory movements exhibit a well-marked transverse 

 arrangement. 



* Zeitschr. f. wiss. Zool., xxxvi. (1881) pp. 1-68 (1 pi.). 



