THE EMBRYOLOGY OF A SCORPION. 121 



Fourth Period. — From the Formation of the Appendages to the 

 Hatching of the Embryo. 



Stage I. — The first stage of this third period shows — as 

 mentioned above — the commencement of some of the most im- 

 portant structures. The embryo, of which a surface view is 

 given in PI. XV, fig. 28, now consists of twelve somites in 

 addition to the head- and tail-segments. These somites are 

 no longer separate thickenings as in the last stage, but have 

 grown close up to one another, and are marked ofi" by narrow 

 grooves. The epiblast extends as a single layer all round the 

 egg. The longitudinal neural groove is well marked and 

 extends the whole length of the body with the exception of the 

 tail-segment. 



The first six somites bear appendages, i.e. the chelicerae, 

 chelae, and four pairs of walking legs. These appendages are 

 simple outgrowths, and are, with the exception of the first two 

 pairs, of approximately equal size. The chelicerae are much 

 smaller, and the chelae somewhat larger than the other appen- 

 dages. The appendages are an outpushing of the epiblast and 

 the outer layer of mesoblast or somatopleure (PI. XVI, fig. 31). 

 They are hollow, the spaces being prolongations of the coelomic 

 pouches. There is at this stage no sign of appendages on the 

 somites behind those bearing the walking legs. 



The embryo has a strong dorsal flexure so that the cephalic 

 segment curves round the end of the egg. This is best seen in 

 longitudinal section (PI. XVI^ fig. 29). The anterior margin 

 of the cephalic segment is deeply cleft in the middle line, the 

 segment being thus divided into two lobes. The lobes are in 

 much the same state as in the last stage, and show no signs of 

 the cerebral invagination from which a greater part of the brain 

 is formed. In the middle line, and a very short way behind 

 the bottom of the cleft, is a circular raised area with a pit in 

 its centre (PI. XV, fig. 28, st.). This pit is the stomodseum. 

 It is seen in section in PL XVI, fig. 29, and is a simple inpush- 

 ing of the epiblast. 



VOL. XXXI, PART II. NEW SER. I 



