48 EIOHAKD EVANS. 



IV. The Development of the Germ Layers, etc. 



The First Embryo. — The sections obtained from the 

 specimen illustrated in the first figure on PI. 5 were not 

 sufficiently good to admit of tlie structure being made out 

 with any degree of accuracy and certainty. For tliis reason 

 it is necessary to commence the description of the formation 

 of the germ-layers from a slightly older embryo, of which 

 four transverse sections are represented in fig. 9 [a, h, 

 and d) on PL 6. 



In the embryo under consideration there is no external 

 yolk, and the blastopore is as yet undivided ; but both the 

 endoderm and mesoderm are already in process of formation. 

 At this stage in the development there are no nuclei in the 

 centre of the yolk. 



The ectoderm consists of a single layer of cells except in 

 front and at the sides of the blastopore on the ventral sur- 

 face where the nuclei are already arranged two deep (PI. 6, 

 fig. 9 a). On the dorsal surface, especially towards the ante- 

 rior end, the ectodermal layer seems to be incomplete. The 

 ectodermal nuclei of the ventral surface are oval in shape, 

 and are arranged close to one another ; while those situated 

 at the sides and on the dorsal surface are circular in shape, 

 and placed at greater distances from one another. The 

 doubling of the ectodermal layer, in front and at the sides 

 of the blastopore, seems to represent the first rudiments of 

 the nervous system, which is always developed from before 

 backwards, a fact which explains the greater condensation of 

 ectodermal nuclei in the position in question than elsewhere 

 (PL 6, fig. 9 a). 



The mesoderm is already in process of formation. It is 

 derived from an area situated immediately behind the poste- 

 rior end of the blastopore (PL 6, fig. 9^). The first somite, 

 already present, has not yet formed a cavity, though the 

 nuclei are arranged in a ring (PL 6, fig. 9 c). In addition 

 to the first somite, the rudiments of the second and third 



