67 



studied with regard to its development. It is to be sincerely hoped that 

 some AustraUan naturahst will take up this matter for a complete study, 

 which will need a rich supply of material and also constant access to 

 living material. 



Fertilization was undertaken on Febr. 25th 1915. The development 

 proceeded at no exceptional speed; the gastrulation took place at the 

 age of ca. 18 hours, while the first metamorphosed specimens were found 

 after 4 — 5 days. The eggs are large, ca. 0.5 mm, red-yellowish and quite 

 opaque. They float at the surface of the water; this is the case 

 also in Phyllacanthus parvispinus, while it is otherwise unknown in Echi- 

 noids. It is evidently due to the fact that the egg contains some fatty 

 substance; though I did not observe this in the living objects, it is easily 

 seen in the preserved eggs that they have a vacuolated structure, and 

 it can hardly be doubted that these vacuoles must contain some fatty 

 substance which makes the eggs lighter than the water. The same vacuol- 

 ated structure was found in Phyllacanthus parvispinus (p. 24 — 25). 



The cleavage is total and perfectly regular. It appears that the blastula 

 is not folded, as is the case in Phyllacanthus. The formation of the mesen- 

 chyme could not be followed in detail. It is seen to begin very early; 

 in the stage represented in PI. XVIII. Fig. 1, which is 6 hours old and 

 apparently either in the 32- and the 64-cells stage (this cannot be definitely 

 ascertained from the sections), one cell is seen lying in the middle of the 

 blastocoel cavity, which must, evidently, be a mesenchyme cell. PI. XVIII 

 Fig. 2, representing an 18 hours old blastula, shows the blastocoel cavity 

 completely filled up by vacuolated mesenchyme cells; they are still un- 

 differentiated and of the same general aspect as the ectoderm cells, which 

 latter do not yet form a regular epithelial layer. The whole of the embryo 

 is full of large and small vacuoles, which must, evidently, undergo 

 divisions together with the cells, as is apparent from the fact that they 

 are much more numerous and, upon the whole, smaller in the older than 

 in the younger stages. (Comp. PI. XVIII, Figs. 1 — 2.) 



A further advanced stage, from a culture 18 hours old, is shown in 

 PI. XVIII, Fig. 3, representing the fully formed blastula. The ectoderm is 

 now a typical, high epitheUum, sUghtly higher in one side, where the 

 gastrula-invagination is to take place. The mesenchyme has undergone 

 a noticeable differentiation, the cells forming now merely a fine reticulum 

 between the vacuoles, which have partly united into some very large 

 ones. Some few vacuoles are still found in the ectoderm. This peculiar 

 structure of the mesenchyme, recalling a parenchymatous plant tissue, 

 is retained during the whole process of the embryonal development, 

 until the metamorphosis has been completed. 



9* 



