304 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



The median depression becomes more decided, and, at the 

 end opposite the procephalic lobes, the blastoderm is produced 

 into a sort of papilla, directed forward. This is the rudiment 

 of the caudal extremity. From the anterior part of the 

 blastoderm there arise, on each side, two papillae, the points 

 of which are directed backward, and which will become the 

 antennules and antennae. The whole of these parts are in- 

 vested by a delicate cuticular membrane, which gradually ex- 

 tends over and invests the whole yelk beneath the vitellary 

 membrane. At the end of the caudal papilla it forms a broad 

 process, produced into setae, which sometimes appear fan- 

 like, sometimes so deeply bifid as to resemble two styles. 



The embryo has now reached what we may term its larval 

 stage, and, in this condition, it leaves the vitellary membrane 

 within which it was inclosed, and lies free in the ovigerous 

 pouch of the parent. At the same time, the caudal extremity 

 enlarges, and straightens itself out, so that no indication of 

 its previous inflexion against the thoracic portion of the blas- 

 toderm remains. The larva thus much resembles a pear 

 (Fig. 79, D, JE), with four processes (2, 3), the antennules 

 and antennae, which have now become much elongated, on 

 one surface. 



The young 3fysis next grows rapidly and undergoes great 

 changes in form : but it is a very remarkable fact, that the 

 primitive integument remains unaltered ; gradually enlarging, 

 to accommodate itself to the increased size of the foetus, in- 



Fig. 70. 



poda) and that of the head of a vertebrate embryo. The procephalic pro- 

 cesses resemble in a remarkable manner the trabecules cranii of the vertebrate 

 embryo ; and the cephalic flexure of the Crustacean or Insect has its analogue, 

 if not its homologue, in the angle which tbe trabecular region of the base of 

 the skull at first makes with the parachordal region in almost all Vertebrata. 



