30 Dynamic Theory. 



This bull next undergoes an inverting process called " invagination, " 

 in which the dark side of the ball is in appearance gradually pushed in- 

 to the interior hollow so as to form a lining for the bright side. The 

 two layers thus form in succession a cup and a bell. Subsequently the 

 more rapid growing of the bright or outer layer of cells turns the bell 

 almost into a globe, having the bright cells for its outside and the dark 

 ones for its inner wall. From these two layers the body of the future 

 animal is developed, the outer layer forming the skin and hard parts of 

 the body, and the inner one, the stomach and internal organs. This 

 gastrula is called the "bell gastrula" or ( Hellenized) "archigastrula. " 



FIG. 45. 



FIG. 46. 



FIG. 45. The gastrula of a chalk sponge (Olynthus) covered with cilia. An external 

 view. 



FIG. 46. A vertical section of same, o. Mouth (and anus). #. Primitive intestinal 

 cavity, e. Exoderm or skin layer, i. Entoderm or intestinal layer. 



In the second division of the holoblastic eggs, the gastrula is called 

 the hood gastrula. It is the type from which all mammals, so far as 

 known, are developed, and has been described already. In it the outer 

 layer of bright cells grows around into a hollow sphere, and the inner or 

 dark layer subsequently lines it by growth upon the inside. 



In the third subdivision, the cleavage is said to be ( 'discoidal, " be- 

 cause when the parent egg undergoes segmentation it does not take the 

 form of a sphere or ball, but that of a disc or plate, lying on top of the 

 nutritive yelk, which is comparatively an enormous ball, often many 

 thousand times greater than the vitalized cell a hen's egg is an exam- 

 ple. The continued segmentation of the cells of the first plate or disc 

 results in the production of a second, a third, and finally a fourth plate 

 or layer, and the various parts of the future animal are derived from 

 these four plates. 



The fourth type of egg cleavage is that called the superficial cleav- 

 age, forming the <l bladder gastrula." In this the segmentation of the 



