198 



NEMERTEA. 



arthropodan embryos. The plate so formed gradually extends 

 itself so as to close over the dorsal surface, and to form a 

 complete skin within the primitive larval skin, which at this 

 period is richly ciliated, though the embryo is not yet hatched 



FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois.) 



A. Side view of an embryo at a very early stage as an opaque object. 



B and C. Two late stages, seen as transparent objects from the ventral surface. 



at. archenteron; m. mouth; pr, d. prostomial disc; po.d. metastomial disc; 

 cs. lateral pit developing in B as a diverticulum from the oesophagus; pr. proboscis ; 

 ms. muscular layer (?); Is. larval skin about to be thrown off; me. mesoblast; 

 st. stomach. 



(fig. 91 C). While these changes are taking place, there are 

 budded off from the invaginated discs a number of fatty cells, 

 which fill up the space between the discs and the archenteron, 

 and eventually form the mesoblastic reticulum. During this 

 stage the rudiment of the proboscis also makes its appearance as 

 a solid process of epiblast, which grows backwards from the 

 point of fusion of the two prostomial discs at the front end of 

 the embryo (fig. 91 C, pr.). A lumen is excavated in it at a 

 later period. The lateral organs or cephalic pits arise in a 

 somewhat unexpected fashion as a pair of diverticula from the 



