140 E. W. MACP.RIDE, 



double membrane which separates the aboral from the axial 

 sinus. 



Hsemal System. 



Before proceeding to describe the development of the genital 

 organs it will be well to detail my observations on the hsemal 

 system, as the composition of the genital rachis is connected 

 with this subject. The view (accepted from Ludwig [11] and 

 Hamann [7] ) which is commonly taught in text-books is that 

 there is a true haemal system of vessels filled in preserved 

 specimens with a deeply staining clot. The aboral sinus is 

 said to enclose one of them, which in its turn surrounds the 

 genital rachis. Another ring-shaped blood-vessel is said to lie 

 close to the dorsal surface of the nerve-ring, and to send 

 branches along the mid-dorsal line of the radial nerves. 

 Finally the ovoid gland has been described as a plexus of 

 vessels connecting these two rings. 



In Amphiura squamata conditions are as follow: — A 

 section of the genital rachis shows no trace of a blood-vessel. In 

 the centre one sees the characteristic primitive germ-cells with 

 large oval nucleus, and well marked body which also stains 

 well. Usually not more than two of these are seen in one 

 section of the rachis. Associated with them are a number of 

 smaller interstitial cells. These form a kind of epithelium 

 over the surface of the rachis which is turned towards the 

 sinus. The outer wall of the latter (i. e. the wall projecting 

 into the coelom) is covered with the ordinary peritoneal epi- 

 thelium. The formation of these interstitial cells can be seen 

 in figs. 7 a and 9 a proceeding contemporaneously with that of 

 the primitive germ-cells. There is no particle of the section 

 which is not accounted for by the nucleus or plasma of some 

 cell. 



That the ovoid gland is no network of lacunae I have already 

 pointed out. In figs. 7 a, 9 a, and 11 this organ can be seen 

 to send out a prolongation which extends on to the upper and 

 outer surface of the nerve-ring. This prolongation becomes 

 thinner and thinner as it proceeds, but there is no trace of any 



