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PHYLUM ARTHROPODA 



The ovum is surrounded by a vitelline membrane, and also 

 by a firm chitinous shell, secreted by the follicular cells, 

 which is often sculptured in a characteristic manner. This 

 shell is pierced by one or more minute holes (micropyles). 

 Through a micropyle the spermatozoon finds entrance, 

 sometimes (as in the cockroach) after moving round and 

 round the shell in varying orbits. 



The ripe egg usually consists of a central yolk-containing mass, sur- 

 rounded by a thin sheath of protoplasm. As is usual in Arthropods, 

 the segmentation is peripheral or centrolecithal. The central nucleus 

 divides up into several nuclei, which, being united by protoplasmic 

 cords, form for a time a central syncytium. Later, these nuclei emigrate 

 into the peripheral protoplasm, which segments around them ; thus a 

 peripheral layer of similar epithelial cells is formed. Some of the nuclei 

 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 cyUndrical 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 coelomic 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 



