64 K. KISHINOUYE. 



In this process of segmenting the mesoderm of the ventral plate 

 shares (PL XIII, fig. 16), and is divided into as many parts as there 

 are segments in the body (jf the embryo. Moreover it divides itself 

 into two longitudinal bands at the median line except at the cephalic 

 and caudal lobes. Thus there is formed in each segment a pair of 

 mesodermic plates. After a while, eacli of these paired mesodermic 

 plates produces a cavity — the cœlom — apparently by its splitting into 

 two layers (PI. XIII, fig. 18). The outer of the two layers is the 

 somatopleure, and the inner the splanchnopleure. The cœlom there- 

 fore consists at this time of a number of paired cavities (PI. XIA , 

 fig. 22), which are separated from one another. Cœlomic cavities in 

 the cephalic and caudal lobes appear only later on. 



Shortly after the formation of the cœlom, a pair of protuberances 

 appear on each segment. They are the first traces of the appendages 

 (PI. XIV, fig, 23, th. app). The order of their appearance cor- 

 responds to the order of appearance of the segments to which thej- 

 belong. The appendages are formed on segments of the chelicera^ 

 and pedipalpi in all the thoracic and the second, third, fourth, and 

 fifth abdominal segments (PL XIII, fig. 20, PL XIV, fig. 22). 

 The cephalothoracic appendages are formed at the lateral ends of the 

 segments, while the abdominal appendages are formed nearer the 

 median line (PL XIII, fig. 20). Tlie abdominal appendages are little 

 round protuberances, and do not elongate as rapidly as other 

 appendages. The first abdominal segment bears no appendages, as 

 Schimkewitch* has correctly observed. This segment is gradually 

 aborted, and is not distinctly visible at tlie time of the reversion of 

 the embryo. The cœlomic cavities of each segment extend into 

 the appendages. 



The foundations of the nervous system are laid soon after the 



* Schimkewitcli — Etude sui- le Development des Araignées, Arch, de Biolog. VI. 



