262 



CRUSTACEA. 



plasma. Eventually the nuclei, each surrounded by a small amount of 

 protoplasm, approach the surface of the egg and arrange themselves 

 regularly round it. The peripheral protoplasm then segments round 

 these nuclei, and thus we have a central core of unsegmented yolk 

 enveloped by a peripheral ring of rapidly dividing cells. In the central 

 yolk, free nuclei are frequently found ; these are the so-called yolk 

 nuclei. Such a type of segmentation is called peripheral or cerilro- 

 lecithal, and is very characteristic of Arthropod eggs. 



Over a particular region of the segmented egg, known as the "ventral 

 plate," the cells begin to thicken; at this region an invagination occurs, 



Fig. iit. — Longitudinal section of later embryo of 

 Astacus. — After Reichenbach. 



Ec, Ectoderm; ;«., mesoderm cells; e.g., cerebral ganglia; s/., 

 stomodseuin ; A., anus; T. t telson; g., ventral ganglia; s.s., 

 Mernul sinus ; pd., proctodasum ; h., heart ; mg., mid-gut ; yolk 

 pyramids dark. 



which represents the gastrula. At the anterior lip of the blastopore the 

 mesoderm appears, being many-celled from the first. Soon the blasto- 

 pore closes ; the cavity of the gastrula thus becomes a closed sac the 



future mid-gut. The cells of this archenteron take up the core of yolk 

 into themselves in a way which early suggests their future digestive 

 function. On the surface of the egg there have aheady appeared 

 ectodermic thickenings,— the so-called eye-folds,— rudiments of the 

 appendages, and of the thoracic and abdominal regions. 



In the later stages invaginations of the ectoderm form the fore- and 

 hind-gut, which grow inward from opposite ends to meet the endoder- 



