IV. Embryology. 



43 



nearly equal radial cells composing a sphere. These cells gradually 

 move away from one another, so as to surround a central cavity filled 

 with a liquid, the segmentation cavity; thus the wall of the 

 sphere, the so-called blastula, consists of a single layer of cells. 

 When this stage is reached, an invagination of half the sphere 

 into the other half takes place, so that it forms a cup or sac with a 

 double wall. The layers of this wall are separated from one another 

 by the segmentation cavity; when, however, as often occurs, it is 

 completely obliterated by the invagination, they lie close together. 

 This stage is known as the gastrula stage. The outer layer of 

 the gastrula is called the epiblast or outer germ-layer; the 

 inner, the hypoblast or inner germ-layer. The cavity of 

 the sac is called the archenteron; its opening the blastopore 

 or gastrula-mouth. 



2. In other cases, of frequent occurrence (Fig. 30), the cells of the 

 blastula are not of equal size, but those which form the hypoblast are 

 larger (richer in yolk) than the rest. Segmentation, invagination, 

 etc., proceed as in case (1). 



Pig. 30. Different stages in the development of the ovum of a Water-snail (Planorbis). 

 1 — 2 Stages in the segmentation, 3 section of a blastula (the large hypoblast cells are seen 

 below), 4 gastrula, en hypoblast, hh segmentation cavity, m mesoblast, o blastopore, 

 r polar-bodies. — After Eabl. 



The early development of a large number of animals takes place 

 in one of these ways; Coelentera (for these see also p. 45), Porifera, 

 Echinoderma, many Worms, Molluscs, one Vertebrate (Amphioxus), 

 Tunioata. 



3. In some cases (Fig. 31) which otherwise adhere to the order 

 described under (2), the cells which form the hypoblast are very 



Pig. 31. Section through the egg of a Chsetopod (Nereis) at different stages of 

 development. In 3, the gastrula is fully formed, but the blastopore (below, to the left) 

 is still open ; in 4 it is closed, and the formation of the mouth (s) has begun, elc epiblast 

 en hypoblast, / oil globules, m mesoblast, s developing mouth. The vitelline membrane 

 is removed in the first three figures, present in the last. — After Gotte. 



