ON THE DIPLOCHOEDA. 399 



as in the figures. The blastula is simply a single-layered 

 sphere with each nucleus in its place, enclosing a spacious 

 blastocoele space. In this respect Phoronis Buskii ap- 

 pears to resemble the species examined by Caldwell (P. 

 Kowalevskii).^ 



The results of Kowalevski (9) and Roule (15) are in accord- 

 ance with what an exact observer would see of the develop- 

 ment of mesoblast in Phoronis Buskii by an examina- 

 tion of transparent embryos. Roule probably observed the 

 anterior mesoblast cells after their origin from hypoblast, 

 and the posterior masses (metacceles) which he called meso- 

 blastic bands. 



In the case of Foettinger (7) his figures and remarks upon 

 them would lead us to suppose that the bodies he describes 

 are neither cells nor nuclei, and do not give rise to the meso- 

 blast; whilst the entire absence of mesenchyme at early stages 

 in sections as shown by Caldwell, and confirmed by myself 

 above, cast grave doubt upon the results of Metschnikoff and 

 Schultze. It seems more natural to make an appeal to the 

 evidence of sections in a case of this kind. 



With regard, however, to the actual development of the 

 mesoblast from the hypoblast there are considerable discre- 

 pancies between Caldwell and myself. He describes and 

 figures a pair of "modified archenteric diverticula," which 

 are evidently identical with the rudiments of the 

 collar coelom as described above. From them he derived 

 the pre-oral coelom. In this I am inclined to think he was 

 misled by the growth backwards of the two horns of the 

 pre-oral ccelom, which he figured in precisely the same posi- 

 tion as above in fig. 37. The lateral masses of mesoblast, 

 instead of growing forwards as he stated to form the pre-oral 

 ccelom, grow backwards and ventralwards to give rise to the 

 collar or tentacular coelom. 



* I am indebted to Mr. A. E. Shipley, of Christ's College, Cambridge, for 

 kindly sending me some original drawings of the earliest stages of P. 

 Kowalevskii, which appear to resemble closely the similar stages as given 

 in this paper. — A. T. M. 



