MUSEUM OF COMPARATIVE ZOOLOGY. 113 



the cells are likewise separated by a transparent wall of the same general 

 appearance. Each of the cells of the 4-cell stage has a nucleus. 



The 8-cell stage, Figs. 8, 9, follows close upon the 4-cell, the addi- 

 tional cells forming by a subdivision of those already existing. This 

 division is generally regular, all the cells being of like size. In some ova 

 smaller cells were observed with larger in the 8-cell stage. The cells 

 have a centrifugal tendency, and a central unoccupied cavity, cav, can be 

 seen enclosed by them. This cavity, which is the cavity of the blasto- 

 sphere, grows in size as the larva matures. There is no solid morula stage, 

 but a segmentation-cavity can be recognized in eggs as young as the 

 4-cell stage. The transparent layer t, which envelops the 8-cell stage, 

 and which is thought to be the same as the thin superficial layer of ear- 

 liest stages, is somewhat diminished in thickness. It is seen to be spread 

 over the surface" of the cells, and to separate the spheres of segmentation 

 from each other.* A day after the eggs were laid they had developed 

 into free swimming spheres, PI. I. fig. 10, covered externally with cilia. 

 These larvse were found in great numbers free in the jars. The egg has 

 developed into a larval stage, which has burst the imprisonment of the 

 capsule, and the blastomeres have arranged themselves on the periphery 

 of a hollow sphere. The superficial layer of cells, still more transparent 

 than the profound, bears long vibratile cilia ; the larva moves readily from 

 place to place. 



At one pole of this larva the blastoderm, or that shell of cells which 

 encloses the cavity, is slightly thickened and more deeply colored than 

 the remainder. This pole is the pole where the invagination to form 

 the archenteron takes place, and it may be said at once that this pole 

 is the seat of the next important change in the growth of the larva. 

 At this point, PI. I. fig. 11, ach., the blastoderm begins to fold inward, 

 forming an invagination, which later becomes the stomach. The posi- 

 tion where this infolding begins is the pole at which the mouth of the 

 gastrula, or, is later situated. At this time in the career of the young 

 Ophiopholis it begins to depart widely in form from that of the genus 

 Ophiothrix, as figured and described by Apostolides. The segmentation 

 of the egg is very similar in the two genera, but the form of the blasto- 

 pheres is somewhat different. The blastodermic cells are very much 



* This condition of the plasmic cortical layer in stages of cleavage as old as the 

 8-cell condition, is thought to indicate that the layer may be something more than 

 a vitelline membrane, although the vitelline membrane may be formed from some 

 portion of it. It is not possible for me to arrive at any good interpretation of the 

 homology of this structure. 



VOL. XII. — NO. 4. 8 



