SYNOPSIS OF THE PRINCIPAL GROUPS. 109 
verse depression is formed in the blastoderm; this is 
the head fold. A longitudinal depression appears to- 
-wards the end furthest away from the head fold—the 
primitive groove. Another depression now makes its 
appearance between the head fold and the primitive 
groove, and, widening backward, encloses the primitive 
groove. This is the medullary groove. The primitive 
groove disappears. ‘The sides of the medullary groove 
grow up, forming the dorsal lamine, the edges of 
which unite together and form the neural canal. A 
tail fold, similar to the head fold but smaller, is formed, 
and the depressions of these two folds unite laterally, 
marking off the embryo. The mesoblast splits in the 
regions of the abdomen and thorax into two lamina; 
the inner, called the splanchnopleure, formed of the 
hypoblast and part of the mesoblast ; the outer, called 
the somatoplewre, formed of the epiblast and part of 
the mesoblast. The two edges of the splanchnopleure 
unite to form the alimentary canal; and the two edges 
of the somatopleure unite, around the splanchnopleure, 
to form the body wall. Thus the body cavity is formed 
by the splitting of the mesoblast ; consequently all the 
organs in it must be formed from that layer, The 
neural canal is formed by an uprising of the epiblast ; 
consequently the main nervous system, as well as the 
integument, is formed from that layer. The alimen- 
tary canal is formed by the hypoblast. 
Pisces. Respiration during the whole of life by 
branchize; fins, supported by fin-rays, on the 
middle line of the body ; heart with one ventri- 
cle, and usually only one auricle; vertebree 
amphiccelous, except in the bony pike, (Lepi- 
