PHYLUM CCELENTERATA 



219 



upwards and outwards, from the long axis of the entire embryo 

 ( rig. 166, A). Their lower (ventral) ends then become divided off, 

 forming a number of small cells, which constitute the rudiment 

 of a true middle cell-layer or mesoderm (A, me.). A kind of in- 

 vagination of the megameres with their mesoderm cells then takes 

 place, resulting in the formation of a cavity — the infundibulum 

 (B, d.) — bounded below by the megameres, now placed horizontally, 

 and above by the mesoderm. The mesoderm gradually retreats to 

 the dorsal surface (C), finally spreading out between the dorsal 

 ectoderm and the infundibulum. At the same time the ectoderm 

 cells bounding the aperture of the infundibulum grow into it so 

 as to line its ventral portion : in this way the atomodaeum (st.) is 

 produced. The remainder of the cavity widens out and becomes 

 the definite infundibulum (d.), and before 

 long sends off four adradial pouches, the 

 rudiments of the canal -system. At the 

 same time a gelatinous layer (Fig. 167, g.\ 

 the mesoglcea, makes its appearance be- 

 tween the ectoderm and endoderm. 



The later processes of development 

 may be described very briefly. The 

 canal-system gradually assumes its adult 

 complexity and the swimming - plates 

 appear. A thickening of the ectoderm 

 on each side of the body gives rise to 

 the epithelium of the tentacle and of its 

 pouch. The muscle-fibres forming the 

 axis of the tentacle (B, me.) are derived 

 from the mesoderm, which also gives rise 

 to the contractile fibres of the meso- 

 glcea (m«.j). The lithites are formed in 

 the ectoderm -cells of the apical pole, but 

 gradually make their way on to the free m 

 surface of the cells, and become supported 

 on four groups of fused cilia. Four outer 

 groups of cilia unite with one another to 

 form the bell (sk.). 



The most noteworthy points in this 

 somewhat complex process of develop- 

 ment are the following : — 



1. The distinction between a purely 

 protoplasmic part of the egg and a yolk- 

 containing portion. In the Hydrozoa 

 and Actinozoa the yolk-material is small 

 in amount and evenly distributed, the egg being described as 

 alecithal or yolkless. In the present instance the yolk is at first 

 accumulated in the centre of the egg, which is thus ccntrolecithal 



Flo. 107. — Two later stages in the 

 development of Callianira. 

 d. infundibulum ; en. endoderm ; 

 r). niesogloea ; me. mesoderm ; 

 *k\ sense-organ ; st. stomodseum ; 

 t. tentacle. (From Lang's Com- 

 parative A natomy. ) 



