144 THE DEVELOPMENT OP 



postoral in its position. Behind this appendage this re- 

 gion becomes confluent with the undifferentiated portion 

 of the thoracico-abdominal area. At the outer portion of 

 this region the nuclei are more closely placed than in the 

 centre, and sections show that in the former portions the 

 cells are more columnar, while in the median line they are 

 more nearly pavement like. The line of demarcation be- 

 tween the two kinds of cells is rather abrupt and indicates 

 the division between the ectoderm of the median line and 

 that which is to give rise to the ventral nervous cord. In 

 the thoracico-abdominal area a differentiation is also tak- 

 ing place, as the abdomen is budding out. This is accom- 

 plished by an infolding near the anterior margin of the area 

 (af) by which the abdomen is separated from the rest of 

 the area, while the ectoderm, thus inpushed, forms the ven- 

 tral surface of both thorax and abdomen. This inpushing 

 does not at first take place clear across the thoracico-ab- 

 domiual area, but begins near the median line and pro- 

 ceeds there more rapidly than at the sides, the result be- 

 ing to form a pouch projecting some little distance into 

 the egg, the walls of which are formed by the ectoderm of 

 the ventral surface. I think that Ishikawa has been mis- 

 led by this development of the abdomen and has inter- 

 preted the pouch thus formed as the proctodseum. Cer- 

 tainly his "pd" in fig. 62 ('85, pi. XXVII) is not the hind 

 gut, but is the beginning of the abdominal fold. In Cran- 

 gon,as we shall see later, the proctodaeum does not appear 

 until some little time has elapsed. In this formation of 

 the abdomen, Crangon agrees closely with Eupagurus. 

 Mayer's fig. 28 ('77, pi. XIV) represents a condition which 

 is frequently seen in cutting sections of the earlier stages 

 of Crangon parallel with the general ventral surface, so 

 far as the thorax and abdomen are concerned ; the amoe- 

 boid cells there fig-ured I have not seen. The scattered 



