80 EICHARD EVANS. 



only is tlie posterior end free of almost, any twisting, but, the anterior end lias 

 so far unfolded itself that only the antennae and oral papillse are situated in 

 front of the curvature. The actual length of the embryo in its folded condi- 

 tion was 17 mm. 



Fjg. 8 ( X 3). — This figure represents the oldest embryo in the uterus, an 

 embryo which is coloured almost like the adult. The body is quite straight. 

 The actual length of the embryo was 27 mm. 



PLATE 6. 



ExGS.9« — 9fl?(X 120). — These figures represent four transverve sections of 

 an embryo slightly older than that shown in Fig. 1, but not as old as the one 

 shown in Fig. 2. There are no nuclei in the yolk, a feature which should be 

 specially noticed. 



Fig. 9 a. — This figure represents a section passing in front of the anterior 



end of the blastopore. The ectodermal layer is already thickening to 



form the rudiments of the brain. 



Fig. 9 b. — This figure represents a section passing through the anterior 



end of the blastopore (b.p.), which is a wide groove devoid of nuclei 



and situated on the ventral surface. 



Fig. 9 c. — This figure represents a section passing t hrough the posterior 



edge of the blastopore, and shows the thickened lips on either side of 



it. The blastopore in the region situated between the sections shown in 



Figs. 96 and 9c is being obliterated by the growing across of cells from 



the blastoporic lips, wliich are not well marked in the region in question. 



The only somite as yet formed is cut across in the present figure, and 



on its inner side are situated a few endodermal nuclei (en.) derived from 



the lips of the blastopore. 



Fig. 9 d. — This figure represents a section passing behind the blastopore. 



The rudiments of the second and third somites are present as groups 



of nuclei. The endodermal nuclei are more numerous in this region 



than they are in the front part of the embryo. 



Figs. 10 a — 10 /fc (x 120). — These figures represent a number of sections 



selected from a series, cut transversely, of an embryo about the same age as 



the one shown in Fig. 2, in which only the first two somites are visible 



externally. The blastopore is divided into two parts, an anterior and a 



posterior. The central cavity of the embryo is occupied by a mass of yolk, 



which protrudes from the blastoporic openings, and to some extent spreads 



itself over the ventral surface of the embryo. The ectoderm of the dorsal 



surface consists of a thin layer of protoplasm with flattened nuclei arranged 



tangent ially to the surface. On the lateral aspects, and on the ventral 



towards the posterior end, the ectoderm is thickened to form the undififer- 



entiated rudiment of the appendages and nervous systeip. Tlie endodermal 



