142 COMPARATIVE ANATOMY 



blastopore closes, and for a while there is no communication 

 between the mesenteron and the exterior. Even before the 

 blastopore is closed, the hypoblast cells forming the walls of the 

 mesenteron begin to absorb the yolk lying outside them. 



They actually engulf the yolk spherules, and soon become 

 greatly distended. It should be noticed that the nucleus 

 of each cell, surrounded by a mass of granular protoplasm, 

 takes up a position nearest to the seat of activity namely, 

 on the outside of the cell nearest to the yolk ; and the yolk 

 spherules which have been absorbed are passed into the 

 interior of the cell, and are aggregated at that end which 

 is adjacent to the cavity of the mesenteron. This process 

 of absorption of the yolk continues until the whole of the 

 yolk is stored up in the walls of the mesenteron, the hypoblast 

 cells becoming enormously long and pyramidal so as to 

 resemble the yolk-pyramids of an earlier stage (fig. 34, F). 



Meanwhile, important changes have been taking place in 

 the embryonic area on the ventral surface. The optic lobes 

 become more distinct, and project as low, rounded swellings 

 from the surface. Behind them, on either side, two pairs 

 of simple bud-like outgrowths indicate the primordia of the 

 first and second antennae, and a little later a third pair of 

 buds, the primordia of the mandibles, is formed behind 

 them. Between the first antennae a shallow depression of 

 the ectoderm marks the commencement of the stomodaeum ; 

 from the first its anterior edge is overhung by a fold, the 

 future upper-lip or labrum. Posteriorly, just in front of 

 the spot where the blastopore closed, the thoraco-abdominal 

 rudiment rises from the surface of the egg as a large median 

 swelling, in the centre of which the proctodaeum is formed 

 as a narrow but deep invagination extending through the 

 abdominal swelling towards the closed mesenteric sac. This 

 embryonic stage, with three pairs of limbs, is considered by 

 many to represent the nauplius larva of other Crustacea, but 

 it is not expedient to attach too much importance to it. 



The further growth of the embryo may be followed in 

 fig. 34, A, C, E, representing ventral views of successive 

 stages of development, and the longitudinal sections, fig. 34, 

 B, D, F, and fig. 35, should be carefully compared with 

 these. It will be seen that new pairs of limbs are successively 

 added behind those already existing, and that each pair soon 



