70 



EMBRYOLOGY 



us what structures may be derived from the ectoderm when it is placed in 

 new environments. If the ectoderm has the ability to differentiate into a 

 certain structure in a certain environment, the ectoderm is said to possess 

 the potency for the formation of that structure. The potencies of any part 

 may be defined as its abilities to differentiate into various kinds of structures 

 in various kinds of environments. 



The anatomy of an amphibian embryo 



In order to make the proposed analysis we must outline the structures 

 present in an early embryo so that we may recognize them in the results of 

 our experiments. For this purpose a schematic diagram of a longitudinal 

 section is given in Figure 30. These same schematic representations of struc- 

 ture will be used in the analytical part which follows this outline. 



The brain and spinal cord show regional differences, and we distinguish 

 forebrain, midbrain, hindbrain, and spinal cord. The forebrain forms the 

 optic vesicles, which differentiate into the retina and optic nerve, while the 

 epidermis forms the lens of the eye. Opposite the forebrain the nasal 

 placodes develop; these form the sensory epithelium of the nose. Also near 



PRONEPHRIC DUCT SPINAL CORD 



NOTOCHORD 



HINDBRAIN 

 OTOCYST, 



SPLANCHNIC MESODERM 



FOREBRAIN 

 NASAL PLACODE 

 EPIDERMAL PITUITARY 

 PHARYNX 



Fig. 30. A highly diagrammatic representation of the structures in the early 

 embryo. The general shape and position of the parts can be recognized in sec- 

 tions of the embryo. The basic plan of all the adult structures is laid down at 

 this time. 



