378 



IIIROSIII OIISHIMA : 



number 1 and sinks down to the bottom at first, and then spreads out 

 in the water. Ova are then laid by female individuals, and, being 

 much heavier than sea- water, very soon sink to the bottom. 



The ovum is slightly flattened at the animal pole, as is known in 

 Cucumaria normani} It measures up to 440 fi across the axis, and 

 about 310// along it. A radially striated, gelatinous coating of the 

 thickness of about 70 11, is distinctly seen, and the jelly-canal running 

 through it opens at the centre of the flattened surface indicating the 

 animal pole. 



In the ovarial tube, the ova are attached to the wall in such a 

 way that the animal pole (" umbilicus ") is directed towards the internal 

 cavity of the tube. The germinal vesicle is situated eccentrically, 

 nearer the animal pole. 



Immediately after being laid, the ovum shows the second matu- 

 ration spindle, the first polar body being found attached at the 

 umbilicus. 



Cleavage. 



The ovum divides quite regularly until about the 32-cell stage. 

 The blastula is not wrinkled on the surface as is stated to be the case 

 in C. frondosa? C. saxicola and C. normani? but smooth and round, 

 with a slightly thicker wall in the vegetative half. The diameter of 

 the blastula is about 335 fi. 



Gastrula. 



On the next morning, invagination begins at the now slightly 

 flattened vegetative pole, from which numerous mesenchyme cells have 

 been immigrating into the blastoccele. 



1) The openings on the genital papilla are much more numerous and may even reach 

 up to several tens. 



2) II. G. Newth, P. Z. S., 1916. 



3) L. des Arts, Bergens Mus. Aarb., 1910. 



4) H. G. Newth, P. Z. S., 1916. 



