BUDDING PROCESSES OF CEPHALODISCUS DODECALOPHVS. 517 



A series of transverse sections of this stage is shown in figs. 40 to 48. 



In fig. 40 is seen the cross-section of the buccal shield. Its ventral wall is greatly 

 thickened, and in the deeper part of the dorsal wall can be discovered a differentiation 

 of nervous tissue. On the left of the middle line is shown a proboscis-pore {p.p. ) in 

 process of development as an invagination of ectoderm. In fig. 41 it opens into the pre- 

 oral coelome. Above the buccal shield is seen a cross-section of the plumes. At an 

 earlier stage they are circular, but the surface nearest the buccal shield is here depressed to 

 form a longitudinal groove. In the mid-dorsal line the mesodermal lining of the ccelome 

 is thickened by an accumulation of nuclei with protoplasmic processes ; these are 

 situated at the anterior end of the sub-neural sinus. In section 41 the pre-oral coelome 

 is still cut ventrally, but dorsally the sub-neural sinus (sn.s.) is seen, covered by the 

 central nervous mass, and containing the sub-neural gland (sn.gl.). The cross-section of 

 this gland is here closely similar to that already described in the adult. In fig. 43, the 

 plumes can be traced into the collar region, and on the right-hand side a second plume 

 is indicated. The collar-cavities lie on either side dorsally, and between them is seen the 

 dorsal blood sinus (d.s.), the direct continuation of the sub-neural sinus in fig. 41. 



In this series the dorsal sinus can be followed as far as the mouth (fig. 44), but in 

 many specimens is seen in the dorsal mesentery shown in figs. 45 and 46. 



From the study of a number of specimens, it is quite clear that the blood system, as 

 in fig. 36, consisting of a median mesenteric sinus and a terminal sub-neural sinus, is 

 invaded by the endodermic sac from the ventral side in such a way that the median 

 sinus is separated by the post-oral part of the sac into a dorsal and ventral blood-sinus, 

 and the pre-oral part (sub-neural gland) effects a similar separation at its base, but at its 

 distal extremity it rests in the sub-neural sinus. 



These relationships are easy to follow in such a stage as shown in figures 40 to 48, 

 and although matters are complicated in the adult by further differentiations, there can, 

 to my mind, be no possible interpretation of the parts other than that given here. 



I emphasise this because my interpretation of the sub-neural sinus and gland was 

 called in question recently by Dr Harmer (2). He would consider the sub-neural sinus 

 as a structure closed on all sides by mesoderm. If such were the case, his suggested 

 homology with the " proboscis-vesicle " of Balanoglossus would have some facts to 

 recommend it, but the structure of the sinus, as depicted as accurately as may be in 

 figures 40 to 44, and later in figures 78 to 84, seems to me to indicate that at these 

 stages (a) there is no trace of a closer mesodermic vesicle ; (b) the sub-neural gland, 

 arising far out, eventually rests freely in the dorsal sinus and its direct continuation, the 

 sub-neural sinus, the dorsal wall of both these sinuses being formed by ectoderm 

 alone. I have not been able in any later stage to detect any structure developed in this 

 region at all comparable to the " proboscis- vesicle." 



So far, therefore, as the study of the buds is concerned, their structure appears 

 to corroborate my former interpretation of the parts as re-stated in answer to Dr 

 Harmer's criticism (6). 



