NOTES ON THE DEVELOPMENT OF THE ARANEINA. 669 



(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. i) has gone with some detail into the history 

 of the early stages; and Ludwig (No. 5) has published some 

 very important observations on the development of the blasto- 

 derm. Finally, Barrois (No. 2) has quite recently 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. Metschnikoff's 

 observations on the formation of the germinal layers and organs 

 accord in most points with my own. 



The development of the Araneina may be divided into four 

 periods : (i) the segmentation ; (2) the period from the close of 

 the segmentation up to the period when the segments commence 

 to be formed ; (3) the period from the commencing formation of 

 the segments to the development of the full number of limbs ; 

 (4) the subsequent stages up to the attainment of the adult 

 form. 



In my earliest stage the segmentation was already completed, 

 and the embryo was formed of a single layer of large flattened 

 cells enveloping a central mass of polygonal yolk-segments. 



Each yolk-segment is formed of a number of large clear 

 somewhat oval yolk-spherules. In hardened specimens the yolk- 

 spherules become polygonal, and in ova treated with hot water 

 prior to preservation are not unfrequently broken up. Amongst 

 the yolk-segments are placed a fair number of nucleated bodies 

 of a very characteristic appearance, Each of them is formed of 

 (i) a large, often angular, nucleus, filled with deeply staining 

 bodies (nucleoli ?). (2) Of a layer of protoplasm surrounding 

 the nucleus, prolonged into a protoplasmic reticulum. The 

 exact relation of these nucleated bodies to the yolk-segments is 

 not very easy to make out, but the general tendency of my 

 observations is to shew (i) that each nucleated body belongs to 

 a yolk-sphere, and (2) that it is generally placed not at the 

 centre, but to one side of a yolk-sphere. If the above conclusions 

 are correct each complete yolk-segment is a cell, and each such 



