T RAG HE AT A. 



357 



ARACHNIDA 1 . 



The development of several divisions of this interesting group 

 has been worked out ; and it will be convenient to deal in the first 

 instance with the special history of each of these divisions, and then 

 to treat in a separate section the development of the organs for the 

 whole group. 



Scorpio nid3B. The embryonic development always takes place 

 within the female Scorpion. In Buthus it takes place within follicle- 

 like protuberances of the wall of the ovary. In Scorpio also develop- 

 ment commences while the egg is still in the follicle, but when the 

 trunk becomes segmented the embryo passes into the ovarian tube. 

 The chief authority for the development of the Scorpionidie is 

 Metschuikoff (No. 434). 



At the pole of the ovum facing the ovarian tube there is formed 

 a germinal disc which undergoes a partial segmentation (fig. 193 bl). 

 A somewhat saucer-shaped one- 

 layered blastoderm is then formed, 

 which soon becomes thickened in 

 the centre and then divided into 

 two layers. The outer of these is 

 the epiblast, the inner the meso- 

 blast. Beneath the mesoblast there 

 subsequently appear granular cells, 

 which form the commencement of 

 the hypoblast 2 . 



During the formation of the blas- 

 toderm a cellular envelope is formed 

 round the embryo. Its origin is doubt- 

 ful, though it is regarded by Metsch- 

 nikoff as probably derived from the 

 blastoderm and homologous with the 

 amnioii of Insects. It becomes double 

 in the later stages (fig. 195). ^ m QVUM QF SCOKI)JON WITH 



During the differentiation of TH E ALREADY-FORMED BLASTODERM SHEW- 

 thp thrpp PmhrvnmV lavpro trip ING THE PAKTIAL SEGMENTATION. (After 



tne tnree emoiyouic layers tne Metsclinikoff .) 



germinal disc becomes somewhat n. blastoderm. 



1 The classification of the Arachnida adopted in the present work is shewn below. 



fScorpionidse. n A ,_.,:.,., (Tetrapneumones. 



JPedipalpi. Aranema. lDi pne iunones. 

 I- ArthrO^astra. (Pseudoscorpionidse. 



i Soiifugfe. III. Acarina. 



PhalangiJffl. 



2 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff 

 doubtfully derives them from the blastoderm cells ; from my investigations on Spiders 

 it appears to me more probable that they originate in the yolk. 



