388 



INVEETEBEATA 



CHAP. 



marked, and four blastomeres situated in the centre of one face are 

 distinguishable by their granular contents. These blastomeres are 

 the rudiment of the endoderm, and the blastula stage may be said 

 to be now completely attained. 



At the next stage the endodermal cells sink inwards, filling up 

 the blastocoele, whilst the other cells meet beneath them, and so the 

 gastrula is formed. One is thus reminded forcibly of the extreme 

 flattening which the blastula of Polygordius undergoes just prior to 

 gastrulation. The blastopore, or aperture left by the in-sinking of 

 the endoderm cells, is almost immediately closed. The face on which 

 it was situated will be called the oral face. 



Two cells, placed symmetrically to the right and to the left of 

 the middle hne, are found at the next stage in the development of 

 Alcyonidium albidum, which also gives rise to a Cyphonautes-like 

 larva; but they have not as yet been observed in the case of 



Fig. 312. — Early stages iu the development of the egg of Membranipora pilosa. 

 (After Prouho.) 



A, stage of sixteen blastomeres seen from the side. B, stage of thirty-two blastomeres seen from the 

 underside. C, gastrula seen from the side. &ijj, blastopore ; end, ceUs which form the endoderm. 



Membranipora, though it is quite probable that they exist there 

 also. In the case of Alcyonidium they help to form larval muscles, 

 which traverse the blastocoele, and these muscles also exist in the 

 larva of Membranipora. These two cells, whose exact origin Prouho 

 could not determine, he calls the mother cells of the mesoderm. 

 They are, however, situated in front of the mass of endoderm, and 

 have, in all probability, nothing whatever to do with the true pole 

 cells of the mesoderm in Polygordius which give rise to the coelomic 

 wall, but are rather to be compared to the mesectoderm of Polygordius, 

 i.e. cells derived from the second quartette of micromeres, i.e. from 

 the ectoderm which gives rise to the blastocoelic muscles in the larva. 

 Coming now to the next changes observed in the embryo of 

 Membranipora, we find that the whole embryo takes on a conical 

 shape, the oral face forming the base of the cone, whilst at the upper 

 end, where the point of the cone should be, a thickening of the ecto- 

 derm becomes visible, which is termed the apical organ and which 

 is homologous with the apical plate of AnneUdan and MoUuscan 

 larvae. On the oral face now appears a wide depression. This is 



