NOTES ON THE DEVELOPMENT OF THE ARANEINA. 105 



In the Tracheata the mesenteron is always considerable, 

 and the proctodseum is always short. The derivation of the 

 Malpighian bodies from the proctodseum is common to 

 most Tracheata. Such organs are not found in the 

 Crustacea. 



With reference to other points in my investigations, the 

 evidence which I have got that the chelicerse are true post- 

 oral appendages supplied in the embryo from a distinct 

 postoral ganglion, confirms the conclusions of most previous 

 investigators, and shows that these appendages are equivalent 

 to the mandibles, or possibly the first pair of maxilla of 

 other Tracheata. The invagination, which I have found, of 

 part of a groove of epiblast in the formation of the supra- 

 oesophageal ganglia is of interest, owing to the wide exten- 

 sion of a similar occurrence amongst the Tracheata. 



The wide divarication of the ventral nerve cords in the 

 embryo renders it easy to prove that there is no median in- 

 vagination of epiblast between them, and supports Kleinen- 

 berg^s observations on Lumbricus as to the absence of this 

 invagination. 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 afliirming the existence of such an invagina- 

 tion in either the Chsetopoda 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, Bobretsky, and Graber, on 

 Insects. They show, however, that the first formed meso- 

 blastic 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, "Mcmoire sur le Developpement des Araneides," ' Ann. 

 Sci. Nat./ series v, vol. xvii, 1873. 



2. J. Barrois, " Recberches s. 1. Developpement des Araignees," 

 • Journal de 1. Anat. e. de la Physiol.,' 1878. 



3. E. Claparede, 'Recherches s. I'Evolution des Araignees,' Utrecht, 

 1860. 



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



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

 'Zeit. f. wiss. Zool.,' vol. xxvi, 1876. 



