156 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The development of the Ctenophora has recently been 

 thoroughly investigated by Kowalewsky and by A. Agassiz 

 (' Memoirs of the American Academy of Arts and Sciences," 

 1874). 



The laid egg is contained in a spacious capsule, and con- 

 sists of an external thin layer of protoplasm, which, in some 

 cases, is contractile, investing an inner vesicular substance. 

 After fecundation, the vitellus thus constituted divides into 

 two, four, and finally eight masses ; on one face of each of 

 these the protoplasmic 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 still 

 smaller blastomeres, which become distinctly nucleated when 

 they have reached the number of thirty-two, and form a 

 layer of cells, which gradually spreads round the large blas- 

 tomeres, 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 blastoderm takes place, which, 

 extending through the middle of the large yelk-masses tow- 

 ard the opposite pole, gives rise to the alimentary canal. 

 This, at first, ends by a rounded blind termination ; but from 

 it, at a later period, prolongations are given off which be- 

 come the canals of the enterocoele. 



At the opposite pole, in the centre of the region corre- 

 sponding with that in which the cells of the blastoderm first 

 make their appearance, the nervous ganglion is developed by 

 metamorphosis of some of these cells. 



The invaginated portion of the blastoderm, which gives 

 rise to the alimentary canal, appears to answer to the hypo- 

 blast, while the rest corresponds with the epiblast. The 

 large blastomeres which become inclosed between the epi- 

 blast and hypoblast in the manner described seem to serve 

 the purpose of a food-yelk ; and the space which they origi- 

 nally occupied is eventually filled by a gelatinous connective 

 tissue, which possibly derives its origin from wandering cells 

 of the epiblast. 



In those Ctenophora the bodies of which depart widely 

 from the globular form in the adult state, the young undergo 

 a sort of metamorphosis after they leave the egg, and have 

 acquired all the essential characters of the group to which 

 they belong. 



As might be expected from their extreme softness and 

 perishable nature, no fossil Ctenophora are known. 



