Primitive Cell-layers of the Embryo. 



327 



woven nightcap from its pulled-out to its cap-like condi- 

 tion, or the cells arrange themselves in two definitely marked 



Fig. 5. 



/5X ^end 



Planula, without orifice, 

 formed by direct growth. 

 (Optical section.) 



Fig. 6. 



Planula with orifice, which 

 has broken through. 

 (Surface view.) 



by a breaking through at one pole, communicates with the 

 exterior. In either case the result is an organic form charac- 

 terized by being constructed of two layers of cells, the inner of 

 which lines a cavity opening to the exterior. This cavity is 

 the primitive gastric cavity ; and the organic form thus 

 characterized may be known as the Planula*. 



The production of such a Planula, recognizable under 

 extreme modifications of non-essential general shape (one of 

 the most common causes of which is the admixture of a large 

 mass of secondary yelk with the original egg-cell), is common 

 to the develojjmental history of all animals above the Protozoa. 

 But after this there is a divergence ; for whilst there is a further 

 development of primitive cells in the Vermes, Mollusks, Echi- 

 noderms. Arthropods, and Vertebrates, in the Coelenterata (in- 

 cluding herein the Sponges) these two layers of cells, the 

 endoderm and ectoderm, remain throughout life as the basis of 

 further histological differentiation, even though in the larger 

 forms the ectoderm may largely develop deep layers of a special 

 muscular or skeletal nature. The series of forms thus branching 

 off from the genealogical tree may be termed DiPLOBLASTiCA. 



The endoderm and ectoderm of the polypes and corals was 

 recognized first by Professor Huxley, who at the same time 



* It may be advantageous to use the term Gasirula for that condition 

 of the Planula when the orifice is present, as Hiickel has proposed sinco 

 the above scheme was drawn up. 



