DEVELOPMENT OF THE OVUM. 351 



may be left behind in the central yolk to form the yolk nuclei, or, what 

 is probably the more primitive condition, these are formed by subse- 

 quent immigration from the blastoderm. 



The next process is the appearance of differentiation among the similar 

 cells of the blastoderm. Over a special area the ventral plate (cf. 

 Astacus} the cells increase in number and become cylindrical in shape ; 

 over the rest of the egg the cells flatten out and become much thinner. 

 In the middle of the ventral plate a slight groove is formed by rapid 

 multiplication of the cylindrical cells. This represents the disguised 

 gastrulation, the open roof of the groove being the much -elongated 

 blastopore. The surrounding cylindrical cells unite over this open roof, 

 the groove usually flattens out, and thus we have formed a two-layered 

 germinal streak which spreads forwards and backwards over the egg, 

 and early exhibits externally transverse division into segments. The 

 upper layer is the ectoderm ; the lower includes the rudiments of both 

 mesoderm and endoderm. 



Meanwhile another very important event has taken place. We saw 

 that while the cells of the ventral plate increased in depth, the remain- 

 ing cells flattened out laterally ; at the point where the two kinds of 

 cells unite, on either side of the ventral plate, a double fold arises. The 

 two folds unite over the surface of the ventral plate, forming a mem- 

 branous arch over it. The internal fold is called " amniotic," the 

 outer "serous," from their resemblance to the similar envelopes in the 

 embryos of higher vertebrates. The folds take no direct part in the 

 development of the embryo. 



We must now return to the germinal streak. The gastrula groove 

 may persist as a tube after closure of the blastopore, but it is usually 

 compressed by the ectoderm, or never exists as a distinct cavity. The 

 greater part of the lower stratum of the germinal streak consists of 

 mesoderm. This becomes divided into successive segments at each 

 side, each containing a primitive ccelomic cavity, perhaps continuous 

 with the gastrula cavity. The endoderm arises as paired clusters of 

 cells, found only at the anterior and posterior ends of the primitive 

 streak. These clusters increase rapidly and form long endodermal 

 streaks, which curve downwards so as to enclose the yolk. The streaks 

 meet and fuse, first ventrally and later dorsally, thus constituting the 

 mid -gut. The yolk nuclei previously mentioned have meanwhile 

 increased rapidly, forming yolk cells which absorb the yolk. These 

 cells are included in the endodermic mid-gut, and there break up. As 

 the endoderm grows round the yolk, it is accompanied by a layer 

 (splanchnic) of the mesoblast. Fore- and hind- gut are formed by 

 imaginations which fuse with the mid-gut. 



In the later stages of development the primitive ccelomic pouches 

 lose their cross partitions, become filled with mesenchyme cells, and 

 practically obliterated. The body cavity of the adult is formed by the 

 appearance of lacunae amid the cells of the mesenchyme. 



The tracheae arise as segmentally repeated invaginations of the ecto- 

 derm. The openings of the invaginations form the stigmata. From 

 the hind-gut arise the Malpighian tubules, which are therefore ecto- 

 dermic. The development of the other organs is similar to that of 

 the Crustacea, 



