Variations in Spinal Nerves of Hyla awea. 291 



Hyla aurea we have a further stage of what Bateson calls back- 

 ward Homceosis of the nerves composing this plexus, in other 

 words that we have here, as in the sacral plexus "an assumption 

 by one member of these Meristic series of the form and characters 

 proper to other members of those series." But, whereas in the 

 sacral plexus, nerves VII. (probably), VIII. and IX. are exer- 

 cising in an increasing degree the functions previously exerted 

 only by nerves posterior to them (viz., IX., X., XI. and XIL); 

 in the brachial plexus, nerve IV. appears to take up new duties 

 previously performed only by II. and III. 



We have seen that in the Urodeles and in Pipa among the 

 Anura there is a nerve anterior to II., viz., the sub-occipital, 

 which passes out as the case may be between the skull and the 

 first vertebra, or else pierces the first vertebra ; also, we found 

 that in Hyla aurea as in the Anura generally this is absent. 



Pipa thus seems to be, in some degree, an intermediate form 

 between the Urodeles and the other Anura. A further 

 stage is seen in Rana, since "the fact that the last spinal 

 nerves to join the brachial plexus in Pipa are the Illrd, 

 while in Rana they are IVth, is again an evidence of backward 

 Homceosis."* 



That in Hyla we have a continuation of the same process is, I 

 think, beyond doubt, since we have an increasing thickness both 

 in III. and IV., concurrent with a decreasing thickness in II., 

 and also a most intimate connection of IV. with the brachial 

 plexus. To sum up the results of these observations on Hyla 

 aurea, in a few words, we have found a forward Homceosis in the 

 saci'al plexus, also a backward Homceosis in the brachial plexus. 

 We may therefore conclude that in the Anura, as represented by 

 the forms studied in this connection, there is an advance- 

 ment in the direction of a concentration of the origin and 

 functions of the spinal nerves towards the central region of the 

 body. 



APPENDIX. 



In addition to Hyla aurea, I have examined a few other forms 

 found in Australia, viz., Helioporus pictus, and Limnodynastes 



* Bateson : Materials for the Study of Variation, p. 143. 



