478 



LECTURE XX. 



up by the side of the rectum, and extending over one side of the 

 branchial sac. 



In these the ova in different stages of development may be seen. 

 \njig. 180., A shows the unimpregnated ovum with the germinal 



Development of Cynthia ampulla. 



vessel and nucleus. In b, the impregnated germ-cell is multiplying 

 itself at the expense of the yolk, and inclosing that by a series of 

 secondary cells. As this process goes on, the yolk, so subdivided and 

 assimilated, takes on a granular surface, as at c, each granule or 

 tubercle having its hyaline nucleus. By the coalescence of the peri- 

 pheral layer of these cells, an external membrane is formed, as d, on 

 the exterior of which are oil-like globules. An albuminous fluid is 

 now interposed between the chorion of the egg and the germ-mass. 

 A filamentary body next begins to be formed from a part of the ex- 

 terior germ-cells forming the basis of the test ; which body bends 

 over the visceral mass, e. This body or process progressively elon- 

 gates, then uncoils itself, liberating the rest of the test with the 

 visceral mass, and becoming a freely vibrating locomotive caudal 

 appendage. In the larva, as it may now be called, f, a shows the 

 anterior part of the body, h what is deemed a rudimental eye, c is 

 the integument. In the farther advanced cercariform larva, G, the 

 inner substance of the caudal prolongation is absorbed, leaving the 

 cellulose sheath empty: the anterior process, «, is produced, and 

 some traces of a definition of visceral parts may be detected. In h 

 the caudal appendage is lost ; the stomach and intestine are recog- 

 nisable. In I the young Ascidian may be recognised as such : a is 

 the branchial orifice with tentacles ; h is the anal orifice ; c is the eye- 

 speck still visible, though fading ; d, the oesophageal nervous collar ; 

 e, the position of a pulsating sinus ;yjy, ciliated circles, which are 

 rudiments of the respiratory bag ; g, g, are muscular bands. 



In the genera PoJyclimim and Amarmiciiim, amongst the com- 

 pound Ascidians, Milne Edwards has observed that the ovum, whilst 



