ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 409 



pendiciilarly to their long axis, and tlio onter parent cells divide 

 again parallel to the same, thus increasing the number of cells at the 

 mouth of the gastrula to eight. About the twentieth hour the blasto- 

 pore begins to shift from its basal position towards the future ventral 

 surface. It comes to occupy the apex of a V-shaped furrow directed 

 towards the velum, and is much reduced in size. The rest of the 

 furrow is occupied by ectoderm-cells, which arrange themselves 

 regularly, circularly at the apex. The diverging arms of the V 

 become parallel, the furrow is deepened at the basal end to form a 

 round opening like that at the apex, only smaller. When the blasto- 

 pore is closed, the ectoderm-cells, which formed the sides and floor of 

 the furrow, constitute the walls of the stomodaeum. The endoderm- 

 cells, resulting from the division of the primitive four, are at first 

 irregularly arranged in the segmentation-cavity, but are finally dis- 

 posed as the walls of a slit-like cavity appearing in the centre of the 

 mass. The endo-mesoderm cells leave their median lateral position, 

 approach one another on the future dorsal side, and dividing at right 

 angles to their long axis form the primitive mesoderm-cells. The ends 

 of the endo-mesoderm cells persist on the dorsal edge of the blastopore, 

 and are forced, by its closure, inwards to form part of the endoderm- 

 lining of the mesenteron. The mesoderm-cells come together in the 

 median longitudinal, dorso-ventral plane, and give rise to two V-shaped 

 rows of smaller cells, which afterwards divide to form double rows. 

 Two lateral swellings on each side of the blastopore, as it becomes 

 ventral, unite as the latter moves forward, and form a median pro- 

 tuberance which developes into the foot. The mechanism in the 

 shifting of the blastopore is traced to the changes in cells on the 

 dorsal side of the embryo. The velum increases greatly ; on each side 

 of the main median band of cilia there is a band of support-cells, the 

 anterior due to the decrease of the original anterior band of cilia, 

 the posterior, a new development specially well marked on the dorsal 

 side of the embryo. The shell-gland appears shortly before the 

 closure of the blastopore as a plate of thickened cells, including most 

 of the dorsal surface posterior to the velum. The cells of the embryo- 

 cap increase in number and decrease in size. Two cells, one on each 

 side of the apical plate, project conspicuously, retaining their rounded 

 ends, which become filled with highly refractive granules, and covered 

 with extremely fine, straight, motionless, radiating hairs. At the very 

 pole a tuft of fifteen to twenty long inactive cilia is formed. To 

 these larger and longer hairs, as distinct from the smaller, probably 

 directive cilia, Dr. Patten would ascribe sensory functions. 



6. From the closure of the blastopore to the formation of the nauti- 

 loid shell. — The endoderm-cells arrange themselves more regularly, 

 though very unequally, round the mesenteric cavity. The body- 

 cavity appears as a space between the walls of the mesenteron and the 

 ectoderm. The further development of the mesenteron and the oeso- 

 phagus is described. The mesoblastic chords grow forward as far as 

 the velum, where the cells become isolated, and elongated with pointed 

 ends. Some mesoderm-cells freed from this anterior extremity of the 

 chord, form a layer on the dorsal surface of the mesenteron, and 



Ser. 2.— Vol. VI. 2 E 



