216 



ZOOLOGY 



SECT. 



preceding types, the micromeres forming the ectoderm, the mega- 

 meres the endoderm, and the ventral edge of the ectodermal 

 investment representing the blastopore. There is, however, no 

 archenteron or gastrula-cavity, and the stage has been produced, 

 not by a process of invagination or tucking-in, but by one of epiboly 

 or overgrowth. A gap which is left between the ectoderm cells 

 at the upper (aboral) pole the pseudoblastopore (Fig. 167, A) 

 soon closes up. 



The endoderm-cells increase in number, and become much 

 elongated and arranged obliquely, their long axes radiating, 



upwards and outwards, from the long 

 axis of the entire embryo (Fig. 167, A). 

 Their lower (ventral) ends then become 

 divided off, forming a number of small 

 cells (A, me.). A kind of invagination 

 and rotation of the megameres 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 

 small cells divided off from them. The 

 latter mainly give rise to the tentacular 

 canals. At the same time the ectoderm 

 cells bounding the aperture of the infun- 

 dibulum grow into it so as to line its 

 ventral portion : in this way the stomo- 

 dseum (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 the mesoderm (Fig. 168, me.), 

 with its gelatinous mesogloaa (#.), makes 

 its appearance between the ectoderm and 

 me, ^0P r ^T endoderm, its cells being derived from 



F \teveio'imienT 1 of ter caiii e anira e ec ^ O( ^ erm ceu<s which migrate inwards, 

 d. infundibulum; en. endo- chiefly in the neighbourhood of the 



derm ; g. mesoglcea ; me. 

 mesoderm cells, derived from 



**; tfc The later processes of development may 

 (From Lang's Comparative An- be described very briefly. The canal- 

 atom?/.) J /.- 



system gradually assumes its adult com- 

 plexity and the swimming-plates appear. A thickening of the ecto- 

 derm 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, w^hich also gives rise 

 to the contractile fibres of the mesogloea (me r ). The lithites are 

 formed in the ectoderm cells of the apical pole, but gradually make 



