208 BULLETIN OF THE 



of the medullary plate is greatly thickened, and the lateral plates are 

 thereby wholly cut off from the proto vertebral plate. The thickening of the 

 medullary plate is the hinder portion of a considerable ganglionic mass, 

 which is the basis for the subsequently differentiated ganglia Gasseri, 

 acusticum, and nodosum. 1 The somatopleural thickening may be traced 

 to a point about 80 fj. farther forward, where the body cavity is no longer 

 expanded. The relations of this thickening to the nephridial organs will 

 be discussed in connection with Stage II. (page 211). 



In a slightly older embryo, measuring 2.34 mm. in length, the condi- 

 tion of the mesoderm is nearly the same as in the one last described. 

 The somatic layer shows a marked thickening (Plate I. Fig. 1, eras. su'jjIk.), 

 which is greatest immediately lateral to the protovertebral plate. An 

 anterior ccolomic chamber is also present. The anterior limit of the 

 thickening is situated, as before, about 0.1 mm. in front of the hinder 

 end of the enlargement which is destined to give rise to the cranial 

 ganglia. The thickening (Fig. 1, eras, so'plu.) of the somatopleure is 

 slightly more pronounced than in the younger embryo. 



The results of this study may be summarized as follows. There 

 exists already at this stage a slight somatopleural thickening, which is 

 maximum along a line immediately lateral to the protovertebral plate. 

 This thickening is associated with a local expansion of the ccelom. It 

 is most pronounced in the region directly posterior to the cranial gan- 

 glionic mass. Posteriorly it is lost in a general lateral thickening of the 

 somatic layer. The location of the thickening corresponds closely with 

 the region in which the pronephros and segmental duct later arise. 

 Whether we have in this thickening the first rudiment of the excre- 

 tory system will be discussed in connection with Stage II. 



1 I may here note that I have been able to make out for the series of spinal and 

 craniai ganglia in Rana, Bufo, and Amblystoma an origin not unlike that described 

 by Beard ('88. pp 166. 183) in Selachii and Aves. and by Schultze ('88, p. 349) 'in 

 Rana. The ganglia are developed from the ectoderm at the lateral margins of 

 the medullay plate (Fig. 3, fnd. gn. spi.). The differentiation of the ganglia is 

 already apparent before the neural tube is infolded A spinal ganglion does not 

 arise as an outgrowth from the neural tube, nor as a separate thickening of indiffer- 

 ent ectoderm, but is differentiated from a first rudiment (Anlage) common to it and 

 to the neural tube. 



