194 PHILIP p. CALVERT. 



pole. Up to this period the embryo has occupied a reverse position 

 to that in which it is found in later embryonic life. Its head end 

 has been much nearer the posterior than the anterior egg pole, and 

 its hind end nearer the anterior pole, Avhile the appendages have 

 been directed towards the dorsal egg surface Cfig. 33E). The man- 

 ner in which it changes its position will now be described. 



Near the point where the amnion cavity was closed by the growth 

 of the serosa, the amnion and serosa fuse together. The fused part 

 tears. The torn right and left halves adhere to the dorsal egg sur- 

 face. By contractions of the serosa, the yolk substance is drawn 

 towards the anterior egg-pole, and this in turn forces the embryo, 

 head first, through the rent. When this turning out is completed, 

 the embryo lies with its head near the anterior pole, the appendages 

 towards the ventral egg-surface, and the hind end of the abdomen, 

 which is still folded ventrally on itself, towards the posterior pole. 

 The ventral egg-surface, which was at first almost straight, is now 

 also convex, owing to the increase in size of the egg-contents. The 

 mandibles and first maxillie lie transversely, the other appendages 

 are directed backwards and somewhat towards the median line. The 

 hind end of the abdomen bears three pieces, the beginnings of the 

 tracheal gills, which conceal between them an invagination, which 

 is the commencement of the hind-gut (fig. 34). 



Brandt has given no detailed account of the remaining phenomena 

 of embryonic development. He merely mentions that the segmen- 

 tation of the body becomes more distinct, the appendages become 

 jointed, teeth appear on the mandibles, the second maxillag unite 

 alono- their inner edges, the eyes appear, the cuticle becomes light 

 brown, the yolk gradually disappears from the outside of the ger- 

 minal band, its remainder being visible in the mid-gut. 



When the embryo has reached its final position in the egg, the 

 germinal band forms merely the ventral surface, the appendages and 

 the greater part of the head of the embryo. The dorsal and much 

 of the lateral parts of the thorax and abdomen are represented solely 

 by amnion and serosa enclosing the remaining yolk in a sack. It is 

 considered probable by Korschelt and Heider (18, p. 803), that the 

 serosa part of this sack thickens, is invaginated into the yolk filling 

 the sack, becomes constricted off, that its cells break up and mingle 

 with the yolk, and that the walls of the yolk-sac are then furnished 

 by the amnion alone. Whether the amnion is converted into the 

 definite dorsal body-wall, or whether this is formed by lateral up- 



