A DESCRIPTIVE SKETCH 



19 



Fig. 4 



Diagrams showing the process of invagination and mesoderm-formation in 

 Urodeles. Each diagram is a median sagittal section on to which the mesoderm 

 of one side has been projected, a, Early stage ; the dorsal lip is well advanced, the 

 ventral lip barely indicated, the sheet of mesoderm is beginning to spread for- 

 wards from the dorsal and lateral lips. &,The tip of the notochord {Ch) is growing 

 forwards beneath the neural plate ; the edge of the mesoderm sheet {p) has ex- 

 tended further forward ; a small blastocoel (F) is still visible ; K, the front of the 

 neural plate, c, d, Further stages ; mesoderm is growing in at the ventral lip ; the 

 mesodermal sheet has extended forwards and downwards, and leaves only a small 

 area unoccupied. The paired rudiments of the4ieart are situated near the growing 

 edge of the mesodermal sheet on each side. (From Vogt, Arch. Entwmech. cxx, 

 1929.) 



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