FORMATION OF ORGANS IN THE FROG. 595 



The process of development may then be briefly recapitulated as 

 follows : 



1. The blastodermic membrane, produced by the segmentation 

 of the vitellus, consists of two cellular layers, viz., an external and 

 an internal blastodermic layer. 



2. The external layer of the blastodermic membrane incloses by 

 its dorsal plates the cerebro-spinal canal, and by its abdominal 

 plates the abdominal or visceral cavity. 



3. The internal layer of the blastodermic membrane forms the 

 intestinal canal, which becomes lengthened and convoluted, and 

 communicates with the exterior by a mouth and anus of secondary 

 formation. 



4. Finally the cerebro-spinal axis and its nerves, the skeleton, 

 the organs of special sense, the integument, and the muscles, are 

 developed from the external blastodermic layer ; while the anterior 

 and posterior extremities are formed from the same layer by a pro- 

 cess of sprouting, or continuous growth. 



