ISOMETRA VIVIPARA. 33 



figures 4 and 5 of plate xiv. In figure 4 the nuclei of the ectoderm still form 

 a simple, nearly regular layer, while the entoderm is fairly advanced in 

 development; in figure 5 the nuclei of the ectoderm are already much more 

 numerous, and not arranged in a simple layer, while the entoderm is in a 

 considerably younger stage of development, the cavity having just appeared. 

 Also, the limitation of the ectoderm cells is more advanced in figure 4 than 

 in figure 5. 



In an embryo in a stage corresponding exactly to that represented in 

 figure 4 there was a distinct indication of cilia in one place. An important 

 differentiation is also beginning to take place in the yolk substance; while 

 the whole of the entoderm and ectoderm is uniformly filled with yolk-grains, 

 some of these are collecting into distinct balls of varying size, lying between 

 the ectoderm and the entoderm (plate xiv, figure 4). These groups of yolk- 

 grains remain a very characteristic feature in the following stages of develop- 

 ment; they generally stain intensively red with eosin, and thus are very 

 conspicuous in sections otherwise stained with hematoxylin. 



This corresponds in some degree to what has been described so very 

 carefully for Ophiura brevispina by Caswell Grave. 23 In this case, however, 

 nearly all the yolk material is transferred from the cells to the segmentation 

 cavity, while in Isometra it is only a smaller part of the yolk that is lying 

 free in the cavity which corresponds to the segmentation cavity, while the 

 cells themselves are still filled with yolk granules. 



This remarkable cleavage-modus (the superficial cleavage) is not unique 

 among Echinoderms. I described in 1894 a similar development in Cucu- 

 maria glacialis; M though the material was insufficient for a complete study 

 of the development (the formation of the entoderm could not be made out) , 

 there is no doubt that this is a case quite analogous to that of Isometra 

 vivipara. Furthermore, I can ascertain that there are still more such cases 

 among Echinoderms. In a viviparous Ophiurid, Amphiura vivipara H. L. 

 Clark, observed at Tobago, British West Indies, during the Carnegie expedi- 

 tion, the development is of the same type as that of Isometra vivipara. Later 

 I hope to give a full report of its development, but this may serve as a 

 preliminary notice of this very interesting case, hitherto unknown among 

 Ophiurids. 



Having thus found three cases of this peculiar type of development, so 

 different from the total, regular cleavage otherwise found in all Echinoderms, 

 so far as hitherto known, I expect that it will be found in several other 

 cases, especially in viviparous Echinoderms. That it is not the rule for all 

 viviparous Echinoderms is proved by the fact that Amphiura squamata, 



23 Caewell Grave. Ophiura brevispina, II. An embryological contribution and a study of the effect 

 of yolk substance upon development and developmental processes. Journ. Morph., vol. 27, 1916, p. 426. 



24 Th. Mortensen, Zur Anatomie und Entwicklung der Cucumaria glacialis (Ljungman). Zeitsch. 

 f. wiss. Zool., Bd. LVII, 1894, p. 721-23, taf. xxxn, fig. 32-36. 



