208 PROr. HTJXLEY ON THE 



body ; aud the reproductive elements are developed in the walls 

 of the gastrovascular canals, and pass into them ou their way 

 outwards. 



The development of the coralligenous Actinozoa has not yet 

 been thoroughly worked out ; but Lacaze-Duthiers has shown 

 that, in Coralliii/m ruhrum and other Grorgonidge, the Morula passes 

 into an elongated, almost vermiform, ciliated Gastrula, which be- 

 comes fixed by one end, and then develops the intermesenteric 

 chambers. It can hardly be doubted that these are formed as 

 diverticula from the basal end of the primitive alimentary canal, in 

 vrhich case the developmental process differs but little, essentially, 

 from that of such a Hydrozoon as Carmarina Tiastata ; and the line 

 of demarcation between the Actinozoa and the Hydrozoa becomes 

 very narrow. 



The Ctenophora, on the other hand, differ somewhat in develop- 

 ment, as in other respects, from the Coralligena. Their develop- 

 ment has been carefully worked out by Kowalewsky and more re- 

 cently by Agassiz. 



The laid egg is contained in a spacious capsule, and consists of 

 an external thin layer of protoplasm, which, in some cases, is con- 

 tractile, investing an inner vesicular substance. The vitellus 

 thus constituted divides into two, four, and, finally, eight masses ; 

 on one face of each of these the protoplasm-layer accumulates, 

 and is divided off as a blastomere of much smaller size than that 

 from which it arises. By repeated division, each of these gives 

 rise to smaller blastomeres, which become nucleated when they 

 have reached the number of 32, and form a layer of cells, which 

 gradually spreads round the large blastomeres, and invests 

 them in a complete blastodermic sac. At the pole of this sac, on 

 the face opposite to that on which these blastoderm-cells begin to 

 make their appearance, an ingrowth or involution of the blasto- 

 derm takes place, which, extending through the middle of the 

 large yelk-masses towards the opposite pole, gives rise to the ali- 

 mentary canal. This, at first, ends by a rounded blind termina- 

 tion ; but from it, at a later period, prolongations are given off, 

 which become the gastrovascular canals. 



At the opposite pole, in the centre of the region corresponding 

 with that in which the blastoderm- cells first make their appear- 

 ance, the nervous ganglion is developed by metamorphosis of some 

 of these cells. 



It is clear that the invaginated portion of the blastoderm, which 



