NOTES ON THE DEVELOPMENT OF THE ARANETNA. 693 



ganglion, confirms the conclusions of most previous investi- 

 gators, and shews that these appendages are equivalent to the 

 mandibles, or possibly the first pair of maxillae of other Tra- 

 cheata. The invagination, which I have found, of part of a 

 groove of epiblast in the formation of the supra-cesophageal 

 ganglia is of interest, owing to the wide extension of a similar 

 occurrence amongst the Tracheata. 



The wide divarication of the ventral nerve cords in the em- 

 bryo renders it easy to prove that there is no median invagina- 

 tion of epiblast between them, and supports Kleinenberg's 

 observations on Lumbricus as to the absence of this invagina- 

 tion. I have further satisfied myself as to the absence of such 

 an invagination in Peripatus. It is probable that Hatschek and 

 other observers who have followed him are mistaken in affirming 

 .the existence of such an invagination in either the Chaetopoda 

 or the Arthropoda. 



The observations recorded in this paper on the yolk cells 

 and their derivations are, on the whole, in close harmony with 

 the observations of Dohrn, Bobretzky, and Graber, on Insects. 

 They shew, however, that the first formed mesoblastic plate 

 does not give rise to the whole of the mesoblast, but that during 

 the whole of embryonic life the mesoblast continues to receive 

 accessions of cells derived from the cells of the yolk. 



Araneina. 



1. Balbiani, " Mdmoire sur le DeVeloppement des Araneides," Ann, 

 Set. Nat., series v, Vol. xvn. 1873. 



2. J. Barrois, " Recherches s. 1. DeVeloppement des Araigne"es," Journal 

 de I'Anat. et de la PhysioL, 1878. 



3. E. Claparede, Recherches s, VEvolution des Araigne"es, Utrecht, 

 1860. 



4. Herold, De Generatione Araniorum in Ovo, Marburg, 1824. 



5. H. Ludwig, "Ueb. d. Bildung des Blastoderm bei d. Spinnen," 

 Zeit.f. iviss. Zool., Vol. xxvi. 1876. 



