20 



Growth in length of the Vertebrate Embryo 



out with considerable accuracy the area over which the main axis 

 of the embryo extends. It has been made quite certain that 

 there is a part of the embryo which is developed, so to speak, 

 in situ on the egg, and a part later in origin which is formed by 

 the activity of the blastopore lip. 



These are the protogenetic and deuterogenetic regions respec- 

 tively. 



Fig. 11. Rana temporaria. 



A, B, C, Eggs to show position of bristle; A', B', C', Longitudinal sections of the 

 same embryos after some days; br bristle; bl blastopore; dl dorsal !ip. 

 Drawings of eggs and tadpoles in which bristles have been inserted to deter- 

 mine the parts of the embryo derived from the primary and secondary growth 

 centres. In A the bristle was inserted 5 mm. anterior to the dorsal h'p of the 

 blastopore at its first appearance. In B and C the bristle was inserted about 

 the same distances in front of the dorsal lip but at later periods. 



By carefully marking various parts at different stages of 

 development we are able to arrive at the following conclusion. 

 The protogenetic area includes : 



Forebrain, eyes, mouth, heart, nasal organs, pharynx, 

 and probably the front end of the notochord and perhaps the 

 mid-brain. 



