246 LILIAN SHELDON. 



characters. But in the next stage the peripheral layer has all 

 become absorbed, and after the removal of the shell the 

 features of the embryo can be clearly made out. Figs. 35 and 

 26, which represent an embryo of this stage seen from the side 

 and front respectively, were drawn from an embryo after it had 

 been preserved. The prseoral lobes are very prominent, and 

 are separated from one another ventrally by a rather shallow, 

 wide median groove ; the antennse are beginning to bud out as 

 small protuberances on their anterior dorsal corners. Along 

 each side of the body is a longitudinal ridge, which is very 

 clearly discernible by its prominence and also by its opaque 

 white colour. This ridge is the origin of the appendages, and 

 is . anteriorly divided into distinct lobes, which are the rudi- 

 ments of the appendages of the anterior segments of the body. 

 The posterior end of the embryo is bent up almost at a right 

 angle to the rest of the body, and at the tip where the lateral 

 ridges meet there is a small papilla, which is perforated by the 

 anus. The mouth is visible, situated on the ventral surface 

 immediately behind the praeoral lobes. Except on the prseoral 

 lobes, the lateral ridges, and the anal papilla, the body of the 

 embryo is a dull yellowish colour, which is due to the yolk 

 shining through the thin ectoderm. 



In a series of transverse sections (figs. 21 a — c) through this 

 embryo the following points are noticeable : 



The ectoderm, except over the praeoral lobes and the 

 appendicular ridges, is a thin layer of flat cells. The cerebral 

 lobes of the nervous system (fig. 21 a, br.) are connected in 

 front of the mouth by a transverse commissure (fig. 21 a, Cer. 

 Com.). The ventral cords are continuous with the brain, and 

 form a pair of longitudinal ectodermic thickenings on the 

 inner ventral angles of the leg ridges (figs. 21 b and 21 c, n. s.); 

 they are not definitely separated from the ectoderm, but are 

 distinguishable from it by the round elements of which they 

 are composed. The mouth opens into a thick walled pharynx 

 (fig. 21 a, Ph.), which is in communication with the gut. The 

 anus is present (fig. 21 c, an.) on a terminal papilla, and is 

 formed by a simple invagination of ectoderm. Immediately 



