60 PROFESSOR E. HAY LANKESTER. 



or body-cavity is involved, and on this as on the equally 

 important question of the persistence of the blastopore 

 Haeckel maintains a discreet though disappointing silence. 



The Discoblastic type. — The essential feature in this type 

 is in the overloading of the egg with food-material. It is led 

 up to by many cases among Mollusca, which present the 

 amphiblastic type so far, that the portion of the egg-cell 

 which contains the unassimilated food-material cleaves once, 

 or possibly twice, but ceases after that to take part in the 

 process of cell-formation and serves only as a reservoir of 

 nutriment. Rarely (as I have suggested in the case of 

 Aplysia, in a paper received January, 1874, and published in 

 1875 in the * Phil. Trans.' by the Royal Society, and as Fol 

 has since recorded in the case of Pteropods) the large seg- 

 ments charged with food-yelk segregate at a later period of 

 development a certain number of cells which give rise to the 

 hypoblast, or as appears from Fol's observations, sometimes to 

 part of the epiblast. This late segregation may be compared 

 to the Periblastic formation of the blastoderm to be described 

 below. Haeckel points out that in the Discoblastic type we 

 have cases (especially among osseous Fish, one of which, a 

 marine Gadoid in all probability, he describes and figures in 

 detail) in which the discoidal cap of egg protoplasm or 

 formative-yelk separates entirely from the food-material, 

 which remains perfectly homogeneous, and never gives rise 

 to any cells, after the separation of the cleavage- patch or 

 disc. The growth of this over the relatively huge mass of 

 food-material he describes minutely, and considers that an 

 ingrowth of the first cap of cells takes place all round its 

 periphery — the well-known ' Randwulst ' of the osseous fish's 

 and chick's development. The result is the formation of a 

 Discogastrula perched on amass of food-material, which mass 

 blocks or fills up as it were the blastopore or * Urmund.' 

 The Discogastrula separated from its mass of food-material is 

 seen in fig. 50. Intermediate stages are admitted by Haeckel 

 in which the food-material is not pure, but still contains 

 formative material, which it separates during the growth of 

 the Discogastrula. The cells thus formed are, he says, 

 partly converted into connective tissue, partly into blood- 

 cells. He regards them as belonging to the endoderm. Such 

 are the cells described by Goette in 1874 in the development 

 of the chick, by Balfour in 1874 in the Shark, and by myself 

 in 1873 in Cephalopods {' Ann. Nat. Hist.,' February, 1873, 

 p. 82). To these I subsequently applied the name of 'auto- 

 plasts ' as opposed to the ' klastoplasts,' which are the pro- 

 duets of the segmentation of the cleavage-disc or cap. 



