found the two separated dcirsally. The velum does not become ciliated until 

 quite late in development, tliough the emhi-yo swims about in the poiicli by 

 means of the cilia of the large ciliated cells which form the head vesicle. 



The shell gland appears on the dorsal surface immediately posterior to 

 the second or transverse furrow as a prominence of ectoderm cells. In the 

 place of this prominence an invagination afterward appears; the margin of 

 the invagination extends rapidly and a thin cuticle, the first indication of 

 the shell, ij secreted by the invaginated cells. As development proceeds 

 the shell becomes a.5ymmetrical, developing more rapidly on the left side 

 than on the right. 



The foot arises as a single median protuberance just postei'ior to the 

 mouth. While it shows no trace of a double origin, it occupies a region 

 along which the blastopore closed, so that really the foot may be considered 

 as having arisen on both sides of the blastopore, tliough the lips of the 

 latter have fused before the former appears. Running from the mouth 

 backward over the median surface of the foot is a row of large ciliated cells 

 resembling those on the dorsal area. 



At the posterior end of the embryo three or four large ciliated anal cells 

 appear, and just ventral to these the distal end of tlie intestine is pressed 

 against the ectoderm. The proctodeal invagination does not occur until 

 late in development. The intestine is a tube with a distinct luhien, its 

 walls being formed of small cells free from yolk. In the course of develoji- 

 ment its central end, where it opens into the cavity between the yolk spheres 

 is carried anteriorly and to the right. Throughout its whole length the 

 intestine is pressed closely against the ectoderm. 



The supra-oesophageal ganglia appear as proliferation of the ectoderm on 

 each side of and dorsal to the mouth ; the eyes are formed in connection 

 with these ganglia as involutions of ectoderm. The ganglia of the two sides 

 are connected by a comjnissure, and from the centre of the latter a nerve 

 runs forward to the centre of tlie apical plate, where there is a ciliated 

 depression in the ectoderm, which I believe is a sense organ. A commis- 

 sure connects the supra-oesophageal ganglion of each side with the otocysts. 

 The latter are formed by involution of the ectoderm of the foot, and the 

 pedal ganglion is formed by delamination from the ectoderm at the sides 

 of the foot. 



Urosalpinx cinerea. 



The breeding habits of Urosalpinx have' been fully described by Professor 

 Brooks.' The segmentation is almost identical with that described by Pro- 

 s* for the oyster, and closely resembles the segmentation of 

 cribed by Bobretzky.= The chief difference between the seg- 

 mcniarion in Urosalpinx and Crepidula consists in the fact that while the 

 four macromeres of Crepidula are equal in size, the four macromeres of 

 Urosalpinx are very unequal, one lieing very much hirger than the other 

 three. Two furrows appear simultaneously and seem to divide the ovum 

 into one large sphere and two smaller ones. Really, however, one of the 

 smaller splieros is not completely separated from the larger one, and soon 

 after fuses with it. This smaller sphere is merely a constricted portion of 

 the larger sphere and contains the nucleus. Thus it is seen that of the two 

 furi-ows mentioned, but one is a true cleavage furrow and it divides the egg 

 into :i larger anil a smaller moiety. At the next stage the smaller moiety 

 divide- into two equal parts, and at the .same time two protuberances, each 

 containing a nucleus, are pushed out from the larger moiety. One of these 



fessor Bri 

 ^;M^sa, as 



3 Preliminary observations on the development of the Marine Prosohranchs. Studies 

 from tlie Biological Laboratory, J. H. U., Vol. 1. 



4 The Embryology of the Oyster. Studies from the Biologieal Uboraiory, J. H. I) 

 Vol. 1. 



5 X. Bobretzky. Studien uberdie Erabryonnle Entwieklungdei Gasteropndeu Archiv 

 f. Mik. Anat., Bd. 13. 



