form ;i cross witli three cells in each arm, the crossing of the arms being 

 exactlv at the centre of the ectodermal area. Two of the arms lie in the 

 anteroposterior axis, two in the transverse axis; one arm is anterior, one 

 posterior, one right and one left. In the next stage the radial symmetry is 

 no longer perfect, since three arms of the cross lengthen so as to contain 

 four cells each, while the posterior arm does not lengthen until later. How- 

 ever, with this exception, the radial symmetry is preserved until fifty-two 

 micromeres are formed ; at this stage the posterior arm lengthens by one 

 cell and the three otlier arms split longitudinally. .\t the same time each 

 of the macromeres, except the one mentioned above as giving rise to the 

 mesoblast, divides as did the later into a larger and a smaller moiety. The 

 three smaller cells take positions on the periphery of the egg in the furrows 

 separating the macromeres, one at each end of the transverse furrow and 

 one at the anterior end of the longitudinal furrow. 



In normal eggs there is not a trace of an invagination at the ectodermal 

 pole, such as has been described for Neritina and Fulga, though eggs which 

 were developing abnormally often showed such an invagination together 

 with other irregularities, e. g., the spreading apart of the macromeres and 

 the formation of large yolk containing cells at the periphery of the ecto- 

 dermal area instead of the small protoplasmic ectoderm cells which are 

 usually formed. 



The gastrula is formed by typical epibole. On all sides the ectoderm 

 cells grow around toward the ventral side at an equal rate; thus tlic 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 Xeritina, 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 singleiow of cells. Later it is composed of several 

 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 sep.arated 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,' but lam not aware Jhat any one has hitherto 



F. Blochmann, Ueber die EntwicklunE rter Neritina flwiiaHUs. Zeit. wiss. Zool., 

 .36. 

 1 McMurrich. J. H. U. Circulars, No. 44, 1885. 



