114 BULLETIN OF THK 



not prevent our accepting the theory that the cavity of the gastrula is 

 formed as Metschnikoflf supposes. 



In Ophiopholis it has already elsewhere been shown that the archen- 

 teron is formed by embole, and the known law of development in other 

 Echinoderms would point to the same method in Amphiura. If there is 

 an invagination of the blastoderm to form an archenteron in Amphiura, it 

 is more masked than in Ophiopholis, and at present it is not possible for 

 me to say whether Apostolides' or Metschnikoff's view of the mode of 

 formation of the archenteron is the correct one. It looks very much as 

 if the epiblast is separated from the reddish layer by a delamination, but 

 it must be remembered that Amphiura is a viviparous genus, and possibly 

 has a highly abbreviated * development in its early history. We may 

 consequently suppose that more or less modification or concealment in the 

 embolic mode in which the archenteron is generally formed in Echino- 

 derms has resulted. It must also be remembered that the majority of 

 gastrulaj of Echinoderms are embolic. My observations support in part 

 Apostolides' statement that the primitive cavity, not however the seg- 

 mentation cavity, of the gastrula is the intestine (" anus embryonnaire ") 

 of the future pluteus. Probably the stomach should have been included 

 with the intestine. The external opening, if such exists, is early closed, 

 and if it is lost, as it may be, from the attached life of the young Am- 

 phiura, is probably always functionless. We have the following state- 

 ment in regard to the anus of the young Amphiura. Apostolides states :t 

 " Dans le stade suivant, ou I'ebauche du tube digestif est completement 

 dessinee, on distingue bien au sommet de I'estomac, au milieu d'un bourre- 

 let, I'oritice anal." This would seem to show that there is an anal open- 

 ing, bui how it is formed cannot be answered so far as observation goes 

 at present. 



In his first paper Metschnikoff recognizes an opening into the cavity 

 of the larva, and considered it as formed by an infolding of the blasto- 

 derm. He was probably mistaken in supposing this opening to be the 

 primary opening, or blastopore. He is believed to have missed altogether 



* The abbreviation in development which leads to the reduction in the arms of 

 the pluteus stage in Amphiura is not believed to cause any great modification or va- 

 riation in the development of the primary plates of the test and arms. One or two 

 writers have brought to their aid, in speaking of the apparent discrepancy in the time 

 of the development of these plates in Amphiura and an Ophiuran with a pluteus, the 

 possibility of modification by abbreviation in the former genus. The argument is 

 deceptive, and should not be given too great weight. 



t Op. cit., p. 208. 



