330 Mr. E. Ray Lankester on the 



three primitive layers of cells, which furnish the original 

 material for further histological differentiation, may be termed 

 Triploblastica. 



In all Triploblastica (Vermes, Echinodermata, Mollusca, 

 Vertebrata, Arthropoda) it appears that of the three layers the 

 outer {ejnblast) gives rise to epidermic structures, sense-organs, 

 and the great nerve-centres ; the mid layer {mesoblasf) to mus- 

 cular tissue, skeletal tissue (varieties of connective tissue and 

 cartilage), blood and lymph, and the walls of the cavities in 

 which they are held ; the innernost layer {hypoblast) to the 

 lining of the gastric or alimentary tract and its diverticula, in 

 the form of glands. The primitive orifice of invagination 

 (mouth of the Plamda) does not persist, either as mouth or, 

 as has been erroneously supposed, as anus, but becomes entirely 

 closed up, and a new mouth and an anus eat their way into the 

 gastric cavity from the exterior, developing thus pharynx and 

 terminal intestine. The origin of the generative products is, 

 as in the Diploblastica, not ascertained to be exclusioely from 

 either epiblast or hypoblast. The communication of the meso- 

 blastic blood-lymph-cavity, or a part of it, with the exterior 

 occurs in all Triploblastica, and is accompanied by an ingTOwth 

 of the epiblast, which, appearing in the simplest worms as the 

 pair of segmental organs or "ciliated excretory tubes," persists 

 in all the subsequent modifications of the type (Echinoderms, 

 Arthropods, Mollusks, Vertebrates). 



Notes to C. — The above generalization must be understood 

 as resting on a limited number of facts, which, liowever, are 

 being daily increased in number. Attention has been already 

 drawn in the notes to B to the frequent masking of the Planula 

 stage and invagination-process in this group as well as in the 

 preceding one. In the early stages of development of the few 

 Vertebrata as yet carefully studied (viz. a few fish, Batrachia, 

 and the common fowl) it is only in the Batrachia that evidence 

 of the invagination, and that in a modified condition (see 

 Strieker's valuable paper in 'Zeitschr. fur wiss. Zoologie,' 

 vol. xi., 1861), is obtained. It is yet a question, on which 

 there is a considerable divergence of opinion, supported in each 

 case by careful observation, whether the mesoblast has uni- 

 formly the same essential origin in the various groups of the 

 Triploblastica. The hypothesis that it has is justifiable in the 

 present condition of knowledge as the simplest. We have to 

 look for a reconciliation of the opinions based upon interpre- 

 tation of observations carried out with different animals, which 

 variously point to the derivation of the mid layer from cells of the 

 epiblast, from cells of the hypoblast, from original cells of the 

 primitive polyplast, or from a new cell-formation in the yelk 



