2SO 



PHYLUM ARTHROPOD A. 



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

 enveloped by a peripheral sphere 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 centro- 

 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, 

 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 



h 



pd 



II! 



s.s g 



FIG. 131. Longitudinal section of later embryo of 

 Astaciis. After Reichenbach. 



EC., Ectoderm; jn. , mesoderm cells; e.g., cerebral ganglia; st. , 

 stomodaeum ; A., anus; 7\, telson ; g., ventral ganglia; s.s., 

 sternal sinus ; pd. , proctodseum ; //., heart ; ing;, mid-gut ; yolk 

 pyramids dark. 



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 already appeared 

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

 appendages, and of the thoracic and abdominal regions. 



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

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

 mic mid-gut. The ear-sac and green gland, and the greater part of the 

 gills, have also an ectodermic origin. From the mid-gut the digestive 

 gland is budded out. The heart, the blood vessels, blood, and muscles 

 are due to the mesoderm. 



