CRUSTACEA. 515 



(Bobretzky). The mouth lies slightly behind the anterior pair 

 of antennae, but distinctly in front of the posterior pair. The 

 other appendages, the number of which at the time of hatching 

 varies greatly in the different Decapods (vide section on larval 

 development), sprout in succession from before backwards (fig. 

 240 B). The food yolk in the head and thoracic region 

 gradually becomes reduced in quantity with the growth of the 

 embryo, and by the time of hatching the disparity in size between 

 the thorax and abdomen has ceased to exist. 



Isopoda. The early embryonic phases of the Isopoda have 

 been studied by means of sections by Bobretzky (No. 498) and 

 Bullar (No. 499) and have been found to present considerable 



FlG. 240. TWO STAGES IN THE DEVELOPMENT OF 



A. Nauplius stage. 



B. Stage with eight pairs of appendages, op. eyes ; at 1 , and at*, first and second 

 antennae; md. mandibles; mx l , mx 2 . first and second maxillae; mxp*. third maxilli- 

 peds ; Ib. upper lip. 



variations. When laid the egg is enclosed in a chorion, but 

 shortly after the commencement of segmentation (Ed. van 

 Beneden and Bullar) a second membrane appears, which is 

 probably of the nature of a larval membrane. 



In all the forms the segmentation is followed by the 

 formation of a blastoderm, completely enclosing the yolk, and 

 thickened along an area which will become the ventral surface of 

 the embryo. In this area the blastoderm is formed of at least 

 two layers of cells an external columnar epiblast, and an 

 internal layer of scattered cells which form the mesoblast and 

 probably in part also the hypoblast (Oniscus, Bobretzky ; Cymo- 

 thoa, Bullar). 



332 



