REVIEW. 



91 



hindgut invagination, all trace of the gastrula aperture and of 

 its connection with the midgut has disappeared. The thoracico- 

 abdominal process has increased to such an extent that its front 

 wall now makes an acute angle — the tail-fold — with the surface 

 of the blastoderm. 



Fig. 7. ^^ 





Fig. 8. 



<~fi/ 



Fig. 7- — Longitudinal section of au embryo iu the sixth (Nauplius) stage 

 (diagrammatised after Boljretsiiy). Tiie endoderm is distinguished 

 from the ectoderm by shading, abd. Thoracico-abdominal process. 

 fg. Foregut. mg. Midgut, hg. Hindgut. 



Fig. 8. — Longitudinal scctiou of a more advanced embryo, showing the 

 increase of the endoderm at the expense of the food-yolk (diagramma- 

 tised after Bobretsliy). The supra-cesophageal ganglia and the nerve- 

 chain are distinguished by dotting, m. Mouth, an. Anus. h. Heart. 

 The other letters as in fig. 7. 



In the sixth stage (PI. VI, fig. 6) the Nauplius-form is com- 

 pletely assumed. The mandibles [Md.) have become Well-defined 

 elevations, and two other pairs of processes have appeared in 

 front of them, the rudiments respectively of the antennules {At. I) 

 and antennae {At. II). The mouth {Oe.) has taken on a semi- 

 lunar form, its convexity being divided backwards, and an ele- 

 vation immediately in front of it {lb.) is the rudiment of the la- 

 brum. The anus {an.) has assumed a definite circular contour, 

 and is now of considerable size. The medullary groove has en- 

 tirely disappeared in front of the mouth, but between that aperture 

 and the abdominal process it is perfectly distinct {R.), and at one 



