90 



JOHNS HOPKINS 



[No. 88. 



cells grow around toward the ventral side at an equal rate; thus the blas- 

 topore is formed in the middle of the ventral side. The mesoblasts are 

 carried around with the ectoderm to the ventral side, where they lie at the 

 posterior edge of the blastopore. The mesoblastic bands are soon separated 

 from the mesoblasts; the latter continue to proliferate mesoderm, which 

 extends forward in the lips of the blastopore. 



The cross above mentioned resembles very much an arrangement of cells 

 figured by Blochmann ' for Neritina, the terminal cells in the transverse 

 arms of which are called by him velar cells. In Crepidula it seems that 

 no part of the transverse arms forms the velum. However, the cells of the 

 posterior arm grow very large, the nuclei are vesicular and stain lightly, 

 and the cells become covered by fine cilia, which protrude through a. thin 

 cuticula ; though at first these ciliated cells lie wholly on the posterior side 

 of the ovum, they move forward in the course of development until they 

 come to lie on the anterior side, and as they increase enormously in size, 

 though they seldom divide, it comes about that they cover the anterior part 

 of the dorsal area, extend around over the anterior end of the embryo and 

 down over its sides. These cells finally form the walls of a large head 

 vesicle. 



The velum appears first on the ventral side, just anterior to the mouth, 

 and consists at first of a single row of cells. Later it is composed of seyeral 

 rows, some of which are adoral, and at least a single row runs posterior to 

 the mouth. It is not completed dorsally until much later, though soon 

 after the definitive mouth is formed the velum splits on each side of the 

 embryo, and about half way between the ventral and dorsal surfaces, into 

 an anterior and posterior branch ; the later continues up over the dorsal 

 surface just posterior to the large ciliated cells; the anterior branch, which 

 is the chief one, turns forward over the sides of the head vesicle, and quite 

 late in development the two arms of the anterior branch meet and fuse on 

 the mid line just in front of the ventral part of the preoral velum. Thus 

 two large velar lobes are formed, one on each side. The posterior branch 

 of the velum appears to be the postoral ciliated band, the anterior branch 

 the preoral ; from the corners of the mouth to the middle of the sides of the 

 embryo the two are fused, while ventrally they are separated by the mouth 

 and dorsally by the whole diameter of the head vesicle. A postoral band 

 of cilia has been described as present in the veligers of several gastropods, 

 and among these Crepidula, 2 but I am not aware that any one has hitherto 

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

 quite late in development, though the embryo swims about in the pouch 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, is secreted by the invaginated cells. As development proceeds 

 the shell becomes asymmetrical, developing more rapidly on the left side 

 than on the right. 



The foot arises as a single median protuberance just posterior 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, though 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 the intestine is pressed 

 against the ectoderm. The proctodeal invagination does not occur until 

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

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

 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 cormeeted by a commissure, and from the centre of the latter a nerve 



~ '/ \ 



1 F. lilochiu;inn, Other die Entwickluiig der Neritina fluriofilis. Zeit. wiss. Zool., 



Bd.36. 

 i MeMurrich. J. 1g\. Circulars, No. 44, 1885. 



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

 from the Biological Laboratory, J. H. U., Vol. i. 



4 The Embryology of the Oyster. Studies from the Biological Laboratory, J. H. U., 



Vol. 1. 



5 N. Bobretzky. Studien iiber die Embryonale Entwicklung der Gasteropoden. Archiv. 



f. Mik. Anat., Bd. 13 



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

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

 sure connects the snpra-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. 3 The segmentation is almost identical with that described by Pro- 

 fessor Brooks* for the oyster, and closely resembles the segmentation of 

 Nassa, as described by Bobretzky. 5 The chief difference between the seg- 

 mentation 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 being very much larger 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 spheres 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 

 furrows mentioned, but one is a true cleavage furrow and it divides the egg 

 into a larger and a smaller moiety. At the next stage the smaller moiety 

 divides 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 

 protuberances is cut off to form a macromere equal in size with the two 

 smaller ones; the other protuberance is a part of the larger macromere and 

 again fuses with it. There have thus been formed by two vertical furrows 

 comparable to the first and second cleavage furrows of Crepidula, three 

 smaller and one larger macromere. 



The antero-posterior axis of the embryo is no longer coincident with the 

 first cleavage furrow as it is in Crepidula, but in order to preserve bilateral 

 symmetry the axis is shifted to one side, so that it passes through the centre 

 of the larger macromere and through the middle one of the three smaller 

 spheres. This new axis crosses the first furrow at an angle of about 45, 

 and the " cross furrow " is in this case a true cross furrow, being transverse 

 to the long axis of the embryo. 



The micromeres are formed very much as in Crepidula. The ectoderm 

 extends posteriorly over the large sphere much more rapidly than it does 

 anteriorly over the three smaller spheres, and the blastopore closes almost 

 directly opposite the formative pole and at a point where the four macro- 

 meres meet in the centre. At this point the definitive mouth afterward 

 appears. 



Owing to great difficulty in cutting sections of the Urosalpinx egg its 

 development was not carried farther. 



