6 COMPARATIVE ANATOMY. 



BJJ}. 1 Consisting at first of a single layer of cells, the blastoderm 

 later on becomes two- and then three -layered. From the relative 

 positions of these, they are spoken of respectively as the outer, 

 middle, and inner germinal layers, or as epiblast, mesoblast, 

 and hypoblast. 2 



The question as to the origin of the germinal layers, on account of its im- 

 portant signification, is one of the most, burning problems in Morphology, and 

 as yet we cannot arrive at any full and satisfactory conclusion on the subject. 

 It may, however, be affirmed with certainty that in all Vertebrates the blasto- 

 sphere passes or did so in earlier times into a stage called the Gastrula. 3 

 One must imagine this form as being derived primitively from the blastula 

 by supposing that the wall of the latter (Fig. 4, BD] became pushed in, or 



FIG. 5. GASTRULA. 

 EM, epiblast ; Ent, hypoblast ; Blp, blastopore ; U, archenteron. 



invaginated, at one part, thus giving rise to a double-walled sac (Fig. 5). The 

 outer wall then represents the epiblast (Ekt], which functions as an organ of 

 protection and sensation, while the inner, or hypoblast (En), encloses a central 

 space, the primitive intestinal cavity (archenteron), and represents the 

 assimilating and digestive primary alimentary canal. The opening of the 

 latter to the exterior, where the two germinal layers are continuous, represents 

 the primitive mouth, and is called the blastopore (Fig. 5, Blp). 



Out of the epiblast arise later the epidermis and its derivatives, 

 as well as the entire nervous system. The latter is formed as an 



1 In meroblastic Vertebrate ova the blastoderm only extends part of the way 

 round the periphery of the ovum (Fig. 3). 



2 The terms ectoderm, mesoderm, and endoderm arc applied to the corre- 

 sponding layers in an adult animal. 



3 The process of the formation of the gastrula may be actually observed at the 

 present day in the holoblastic ova of certain Fishes (see note, p. 5) and of Amphibia, 

 and the same process essentially occurs in the meroblastic ova of other Fishes, though 

 it is here more difficult to recognise. In the case of the Amniota the difficulty is 

 still greater, but although we have no direct proof of the existence of a gastrula 

 stage, the intimate connection of the developmental processes throughout the animal 

 kingdom renders it a priori certain that the gastrula is represented in them. 



