424 



INVEETEBEATA 



CHAP. 



caud 



the anterior mass rest, directly, the mass of granular cells forming the 

 still solid rudiment of the primary stomodaeum. From the sides of 

 the anterior mass two solid outgrowths are developed, which are the 

 rudiments of the genital organs, i.e. of vitellarium and germarium ; 

 for in Eotifers, as in Platyhelminthes, a portion of the genital rudiment 

 consists of rudimentary ova, which serve as food to the few ova 

 capable of development, and is termed the vitellarium. 



The ectoderm cells in the dorsal region, just behind the " apical 

 plate," multiply rapidly and project inwards as a solid invagination. 

 This is the first rudiment of the brain. One day later a second, 

 larger invagination of the ectoderm takes place in the centre of 

 the apical plate. It is nearly solid, but a slight hollow appears 



at the surface which soon flattens 

 out. This second ectodermal in- 

 vagination is the second rudiment 

 of the brain ; it impinges on the 

 first, which covers it as a kind of 

 sheath (Fig. 336). 



Soon after, the inner stomodaeal 

 rudiment assumes the form of an 

 oval body, in the centre of which a 

 fine slit-like cavity appears, which 

 is in direct communication with the 

 secondary stomodaeum and mouth. 

 The primary stomodaeum is now seen 

 to be the rudiment of the peculiar 

 pharynx termed the mastax, char- 

 acteristic of the Eotifers. Some 

 portions, however, of this stomodaeal 

 rudiment remain over after the 

 formation of the pharynx, and at a 

 later stage give rise to the salivary 

 glands. The cavity in the pharynx 

 grows wide, and it is curved in such a way as to leave a large median 

 hump on its floor. The hump becomes covered with a thick cuticle, 

 and this cuticle is the first indication of the trophi or jaws. 



The head region now begins to grow rapidly. Indeed, the develop- 

 ment might be said to be characterized by an alternate growth of 

 the head and foot. The growth takes place in the middle of the 

 apical plate and forms an elevation which is the rudiment of the 

 proboscis. The remainder of the apical plate flattens out and forms 

 a shelf around this elevation. On this shelf at a later period the 

 trochal cUia are developed, and the place where they will arise is now 

 indicated by two semicircular strips of cuticle. This shelf is therefore 

 the rudiment of the trochal disc (Fig. 337, A). 



The cells forming the front endodermal rudiment lose their outlines 

 and form a syncytium ; they then develop a cavity in their interior 

 which is the cavity of the mid-gut. About the same time, on the 



FiQ. 335. — Ventral view of embryo of 

 Ccdlidina russeola somewhat older 

 than those represented in Pig. 334. 

 (After Zelinka. ) 



ap, rudiment of apical plate ; caud, rudiment 

 of tail ; m, mouth. 



