80 



from the rest of the yolk and lies on the periphery, where 

 it continues to divide, and so forms a cap of blastoderm 

 cells, no cleavage cells remaining in the food yolk. The 

 starting point coiTesponds to the point where invagination 

 takes place later, that being at the posterior end of the 

 ventral side of the embryo. 



After the formation of the blastoderm a thickening 

 is formed, corresponding to the future ventral side. This 

 thickening is the germ disc. Shortly afterwards, three 

 divisit)ns of the germ disc make their appearance. Two 

 anterior paired portions (PI. lY., fig, 6, a.m.) represent the 

 formative region of the mesoderm; a median thickening 

 [end.) situated posterior to, and between these, represents 

 the fundament of the endoderm. The germ band is next 

 formed by a probable forward growth of the mesoblast 

 rudiments below the ectoderm, the ectoderm increasing in 

 thickness. Three pairs of buds arise ; these are the 

 rudiments of the limbs, and this stage (PL lY., fig. 7) 

 corresponds to the Xauplius stage. Behind these rudi- 

 ments, and in front of the anal aperture, is a mesoblastic 

 area, termed the formative area (/.-.), from which the 

 remaining segments of the body will develop. The 

 arrangement of the mesoderm cells in the formative area 

 is extremely regular. 



At the beginning of the formation of the mid-gut a 

 number of cells (vitellophags) leave the endoderm and 

 wander inwards ; they do not take any part in the forma- 

 tion of the mid-gut, but assist in the disintegration of the 

 yolk. The mid-gut is formed from two layers of cells 

 which arise from the endoderm rudiment ; these lie beloAv 

 the germ band, and gradually grow round the yolk, each 

 being concave on its inner surface. By means of a ventral 

 median piece they unite in the anterior region, and tinally 

 enclose the yolk by growing round to the dorsal side, so 



