62 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



latter species are 0.2 mm. in diameter, while those of Amphiura squamata 

 are about 0.1 mm. 42 



It is not the size of the egg that is the main reason of the meroblastic 

 cleavage. It is true, the eggs of Abatus cavernosus and Amphipneust.es kcehleri 

 are about 1.2 mm. in diameter and those of Cucumaria glacialis 1 mm., and 

 thus of a very considerable size, but the eggs of Hypsiechinus coronatus are 

 only 0.5 mm., not larger than those of Heliocidaris crythrogramma (Valen- 

 ciennes), which has a total and regular cleavage. 43 Similarly the eggs of the 

 meroblastic Amphiura vivipara are only 0.5 mm., while those of Ophioderma 

 brevispinum (Say) with total, regular cleavage are 0.3 mm. 44 and the eggs of 

 Isometra vivipara are only 0.3 mm. in diameter, very inconsiderably larger 

 than those of Antedon adriatica with total, regular cleavage, while those of 

 Antedon bifida are, according to Wyville Thomson, 45 even 0.5 mm. or almost 

 twice the size of the eggs of Isometra. It can be said only that the facts 

 hitherto known tend to indicate that a meroblastic cleavage is probably the 

 rule in such viviparous Echinoderms as have large and yolk-laden eggs, while 

 in those viviparous forms with small eggs, less rich in yolk, the cleavage 

 remains total and regular, as typical in Echinoderms. 



3. THE FORMATION OF THE ENTODERM ; THE GASTRULA. 



It is rather surprising that there should prove to be considerable difference 

 in the formation of the entoderm in Antedon and Tropiometra. In both of 

 them a regular invagination takes place, but in Tropiometra cells are wandering 

 into the blastoccel, presumably from various areas of the ectoderm, before 

 invagination takes place. These cells lie loosely in the blastoccel cavity, like 

 mesenchyme cells. Then the invagination takes place and all the loose cells 

 unite with the archenteron. The entoderm cells thus appear to be of double 

 origin. In Antedon no such inwandering of cells from the ectoderm takes 

 place, the entoderm originating alone from the invagination. 



It is, of course, impossible to say which is the general course of entoderm 

 formation in Comatulids so long as our knowledge is confined to Antedon 

 and Tropiometra each having its own modus. The third species in which 

 entoderm formation has been studied, Isometra vivipara, differs very markedly 

 from the two other forms in this regard, the entoderm being formed by a 

 gradual differentiation, a sort of delamination, no invagination taking place. 

 That this is a consequence of the meroblastic cleavage is evident enough, and 



42 It is a curious fact that there is no direct statement of the size of the egg in any of the rather numerous 

 papers dealing with the embryology of Amphiura squamata. Only Fewkes gives the size of the blastula as 

 0.15 mm. and MacBride states that the earliest larval stage examined by him was 0.2 mm. 



43 Th. Mortensen. Preliminary note on the remarkable shortened development of an Australian sea- 

 urchin, Toxocidaris erythrogrammus. Proc. Linn. Soc. N. S. Wales, XL, p. 204, 1915. 



14 Caswell Grave. Ophiura brevispina. Memoirs from the Biol. Laboratory of the Johns Hopkins 

 University, iv, 1900. Idem., Ophiura brevispina. II. An embryological contribution and a study of the 

 effect of yolk substance upon development and developmental processes. Journ. of Morphology, 27, 1916. 



45 Wyville Thomson. On the embryogeny of Antedon rosaccus Linck (Comatula rosacca of Lamarck), 

 Phil. Trans., vol. 155, p. 519, 1865. 



