EMBRYONIC DEVELOPMENT. 17 



blastula hearing a general resemblance to a biconvex lens (Fig. 6 D). 

 A more active division of the cells of the animal (later aboral) pole 

 can now be remarked, while, from the vegetative pole, four entoderm- 

 cells wander into the blastocoele (Fig. 6 E). As soon as the 

 entoderm-cells have shifted inwards, the surrounding ectoderm-cells 

 seem to join together. This type of entoderm-formation may be 

 traced back to polar ingrowth. A shallow depression in the region 

 of the vegetative pole is often noticeable during these processes. 



After the ectoderm has completely closed, the four entoderm-cells 

 multiply, their derivatives becoming grouped so as to enclose a small 

 slit-like cavity (archenteric cavity, Fig. 6 F). This cavity is usually 

 obliterated later, the entoderm-cells multiply still more, and finally 

 yield a mesenchyme or embryonic connective tissue (the inner mass, 

 central tissue, " Fiillgewebe," Fig. 6 G, z), which completely fills 

 the blastocoele. This tissue, which seems to be of little impor- 

 tance in the later development, playing merely a passive role, 

 soon assumes a reticular character (Fig. 9 Z?, /). While these 

 processes have gone on within, two rows of cells at the edges of 

 the biconvex embryo have become distinguished by their larger size 

 (Fig. 6 E and F) ; one of these rows at a later stage becomes very 

 conspicuous, forming the rudiment of the ciliated ring or girdle of 

 the larva, and is known as the corona or velum (Fig. 6 G, c). 



After the larva has attached itself, the central tissue gives rise to the meso- 

 dermal portion of the first sedentary individual (primary zooecmm), i,e., to 

 the parietal mesoderm-layer on the inner side of the endocyst, and to the 

 outer layer of the sac-like rudiment of the polypide (Fig. 12, b, p. 29). The 

 rest of the central tissue unites with the cell-mass yielded by the degeneration 

 of the larval organs to form a spherical mass (the so-called brown body), which 

 is composed of remains of cells, particles of food-yolk and detritus, and which, 

 when the first polypide develops, is used up as food material. 



The central tissue is derived direct from the primary entoderm of the 

 gastrtila-stage. This germ-layer does not, as a rule, separate into mesoderm 

 and definitive entoderm. BARROIS (No. 7), indeed, maintains that such a 

 separation does take place in Lepralia unicornis, the mesoderm there becoming 

 arranged in paired mesoderm-bands. His observation, however, has received no 

 further confirmation through researches made in connection with various other 

 genera. In those few forms in which an alimentary canal is developed, there 

 must of course be a separation of the mesoderm from the definitive entoderm. 

 In quite early stages (corresponding to that in Fig. 6 F), a separation of 

 the mesoderm from the entoderm \vas observed by PROUHO (No. 28b) in the 

 embryos of Alcyonidium and Membranipora pilosa (Cyphonautes'), but no further 

 details as to the rise of the mesoderm could be observed. 



The embryo which originates in this manner (Fig. 6 G), in which 

 two body-layers (ectoderm and central tissue = primary entoderm) 



