562 E. W. MAOBRIDE. 



posterior portions, and soon afterwards the left anterior por- 

 tion had assumed the rosette form characteristic of the h)-dro- 

 coele. Metschnikoff expressly states that the right anterior 

 sac often likewise assumed the same form, and this is the first 

 clear statement that I have been able to find in the litei*ature 

 that the hydrocoele rudiment is paired. Miiller's figure alluded 

 to above shows the same thing in Piute us paradoxus, but, 

 as already mentioned, he gives no description of it. 



Further researches on the development of Amphiura 

 squama ta were made by Ludwig (16), Apostolides (2), 

 Fewkes (10), Carpenter (7), Russo (29), and myself (17). 

 Ludwig's work has reference to the development of the 

 skeleton ; amongst other things he showed that the ossicle 

 termed a "vertebra" is formed by the concrescence of two 

 calcareous plates, so tliat its homology with a pair of ambu- 

 lacral plates in an Asterid is proved. Apostolides dealt with 

 the whole development, and according to him the endoderm 

 is formed by delamination, not, as in other Echinoderms, by 

 invagination. This extraordinary statement is reaffirmed by 

 Russo, who also deals with the whole development. Russo 

 asserts further that the coelomic cavities arise as spaces ia the 

 mesenchyme; but he utterly fails to confirm Metschuikoff's 

 observations as to the division of the right coelomic sac into 

 anterior and posterior portions, and the occasional assumption 

 of a five-lobed form by the anterior half. Russo's results are 

 at variance with what is kuown of the early stages of 

 development of all other Echinoderms; and since he failed to 

 see what Metschnikoff was able to make out in the embryos 

 of Amphiura squamata his results have been received 

 with general scepticism, but cannot, of course, be formally 

 denied till some other investigator has the patience to re- 

 examine the early stages of development of this species. These 

 stages are opaque and difficult to obtain, hence, no doubt, 

 the unsatisfactory nature of our knowledge on the subject. 

 Fewkes, who had very little material of the early stages at 

 his disposal, deals chiefly with the development of the adult 

 skeleton, but made out also the ectodermal origin of the 



