Notes on the Development of the Aeaneina. By 

 F. M. Balfoue, M.A., F.R.S., Fellow of Trinity 

 College, Cambridge. With Plates VIII, IX, 

 and X. 



The following observations do not profess to contain a 

 complete history of the development even of a single species 

 of spider. They are the result of investigations carried on 

 at intervals during rather more than two years^ on the ova 

 of Afigelena lahyrinthica ; and I should not have published 

 them now, if I had any hope of being able to complete them 

 before the appearance of the work I am in the course of 

 publishing on Comparative Embryology. It appeared to me, 

 however, desirable to publish in full such parts of my 

 observations as are completed before the appearance of my 

 treatise, since the account of the development of the Aran- 

 eina is mainly founded upon them. 



My investigations on the germinal layers and organs have 

 been chiefly conducted by means of sections. To prepare 

 the embryos for sections, I employed the valuable method 

 first made known by Bobretsky. 1 hardened the embryos 

 in bichromate of potash, after placing them for a short time 

 in nearly boiling water. They were stained as a whole 

 with hematoxylin after the removal of the membranes, and 

 embedded for cutting in coagulated albumen. 



The number of investigators who have studied the de- 

 velopment of spiders is inconsiderable. A list of them is 

 given at the end of the paper. 



The earliest writer on the subject is Herold (No. 4) ; he 

 was followed after a very considerable interval of time by 

 Claparede (No. 3), whose memoir is illustrated by a series 

 of beautiful plates, and contains a very satisfactory account 

 of the external features of development. 



Balbiani (No. 1) has gone with some detail into the 

 history of the early stages; and Ludwig (No. 5) has pub- 

 lished some very important observations on the development 

 of the blastoderm. Finally, Barrois (No. 2) has quite re- 

 cently taken up the study of the group, and has added some 

 valuable observations on the development of the germinal 

 layers. 



In addition to these papers on the true spiders, important 

 investigations have been published by MetschnikofF on 

 other groups of the Arachnida, notably the scorpion. 

 Metschuikoff's observations on the formation of the ger- 



