DEVELOPMENT OF CUCUMARIA. 633 



solution of hsematein followed by orange-G as a plasma stain. 

 The figures are all from camera-lucida outlines. 



The Living Young. 



Spawning in every case has occurred in the night, and generally 

 near midnisht. On the one occasion on which a successful 

 culture resulted, males and females of C. iiorma7ii, living together 

 in the same tank, began to spawn within a few minutes of one 

 another. In other cases isolated individuals of both sexes have 

 spawned during the night ; but I could never succeed in fertilizing 

 the eggs so shed by adding sperm-suspension to the water in 

 which they were. My remarks about the living larv^ refer to 

 the above-mentioned culture. 



The newly-shed eggs (PI. I. fig, 1), taken from among the 

 tentacles of the female, are undergoing, or have just completed, 

 their second maturation division. They are flattened at the 

 poles — especially at the animal pole, which tends to float upper- 

 most- — and enclosed in a striated follicular jelly from which the 

 follicle- ceils have been cast off. No definite micropyle can be 

 made out, but the umbilicus of the follicle, situated at the animal 

 pole of the egg, almost certainly has the function of a micropyle, 

 since sperms ax-e unable to penetrate the jelly. The polar bodies 

 project into the umbilicus. I did not observe the entrance of 

 the sperm into the egg. (PI. I. fig. 2.) 



In the segmentation which follows there is nothing of that 

 ideal i-egularity that has been described in the case of the egg of 

 tSi/napta *. The first two cleavage planes are usually meridional, 

 and divide the egg into four equal blastomeres, which may then 

 rearrange themselves in relation to the original egg-axis (PL I. 

 fig. 3); but in some eggs the first two blastomeres do not divide 

 simultaneously. The third cleavage is equatorial. Subsequent 

 divisions, so far as I am able to discover, do not follow any 

 orderly scheme. They result in the formation of a morula 

 (PI. I. fig. 4), which gives rise to a wrinkled blastula of the type 

 first described by Masterman, in Cribrella (10), and, later, by 

 Gemmill, in Solaster (5) and Forania (6). At this stage (PI. I. 

 fig. 5) the embryo acquires cilia, and soon after emerges from the 

 egg-membrane and begins to rotate slowly at the bottom of the 

 culture tank. Gastrulation is marked externally by the smoothing 

 out of the superficial wrinkles, and by elongation of the larva. 

 The fully-formed gastrula is more opaque at its anterior (prse- 

 oral) end, and in swimming this end is always iipwards. The 

 larvae now swim just below the surface of the water, and rotate 

 slowly about their long axis — in a counter-clockwise direction as 

 seen from above t. A constriction appears soon after this, in 



* This irregularity is due, I think, in great part, to polj'spermy and to unnatural 

 conditions in the laboratory. In a few individuals of C. saxicolal have seen perfect 

 symmetry of cleavage up to tlie 16-cell stage. 



t The blastulaj of C. saricola rotate clockwise ; but after gastrulation the direc- 

 tion of rotation is, in the majority, reversed. I have no note about the direction of 

 rotation of the C. normani blastula. 



Peoc. Zool. Soc— 1916, No. XLIV. 44 



