Development of the Spiders. 199 



This being stated, we may pass to the formation of the 

 embryo. 



The first observer who occupied himself seriously with this 

 subject did not go so far as to the formation of the ventral 

 embryonic band ; he only described the thickening of the 

 blastoderm, and then passed without any transition to the stage 

 of the young spider rolled up in the egg, leaving an enormous 

 hiatus. 



This gap was in part filled up by Claparede, who greatly 

 added to our knowledge on this subject by describing, the 

 development of the embryonic band, its subsequent division 

 into ventral pads, and its segmentation into zonites of three 

 kinds — thoracic, abdominal, and postabdominal. Neverthe- 

 less even he saw only half the phenomena ; his description 

 omits, between the last stage of the embryonic bands and the 

 young spider, a second series of important phenomena which 

 have hitherto remained unknown. 



In what follows I shall divide the subject into three parts : — 



1. The stage of the embryonic bands, described by Claparede, 

 as to which I have only a few supplementary notes to add ; 



2. The Limuloid stage, hitherto unknown, and which I shall 

 have to describe completely; 3. The stage of the young 

 spider rolled up in the egg, already well investigated by 

 Herold, but which we shall have to reconsider from the point 

 of view of the lamellae. 



I. Embryonic Band. 



If we make a section of the egg at this stage, we see that 

 Claparede's ventral pads are really composed of two lamellae : 

 — ] , the external simple one, which extends over the whole 

 surface of the egg ; and 2, the intermediate one, arranged in 

 two cords, formed of several rows of embryonic cells : these 

 cords exactly correspond to the ventral pads ; and 1 reserve to 

 them the name of germinative bands. These germinative 

 bands exist throughout the whole length of the egg. They are 

 derived from the scission of an originally continuous meso- 

 dermic band, but do not present the same arrangement through- 

 out : in the abdominal region they are thin and of but small 

 extent ; in the future thoracic region, on the contrary, they 

 are much larger, and begin to show a division into a central 

 part (the nervous bands) still adherent to the external lamella, 

 and a peripheral part, which is purely mesodermic. In front 

 the two germinative bands pass to two projections of a 

 circular form, direct prolongations of their nervous portion, 

 which will form the cerebral lobes ; these are the representa- 

 tives of the procephalic lobes of Claparede and Huxley 



H* 



