NERVOUS SYSTEM. 



411 



Hatschek states that there takes place an invagination of a median part 

 of the skin between the two ventral cords, for the details of which I must 

 refer the reader to Vol. II. p. 410. He has made more or less similar 

 statements for the earthworm, but his observations in both instances are 

 open to serious doubt. 



FIG. 244. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF 



AGELENA LABYRINTHICA. 



st. stomadteum ; gr. section through semi-circular groove in procephalic lobe ; 

 ce.s. cephalic section of body cavity. 



Full details as to the development of the nervous system in 

 the Crustacea are still wanting ; a fairly complete account of 



P> 



FlG. 245. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYO OF HYDROPHILUS. 



(After Kowalevsky.) 



A. Transverse section through an embryo in the region of one of the stigmata. 



B. Transverse section through an older embryo. 



vn. ventral nerve-cord ; am. amnion and serous membrane ; me. mesoblast ; tiie.s. 

 somatic mesoblast; hy. hypoblast (?); yk. yolk-cells (true hypoblast); st. stigma of 

 trachea. 



