80 



EMBRYOLOGY 



ectoderm (Kowalevsky et Marion, No. 87). In it are 

 secreted the first calcareous spicules (sclerites) (v. Koch, Nos. 

 82 and 84, Kowalevsky). These arise as small, highly 

 refractive bodies (sp) within the mesodermal elements, 

 which resemble migi'atory cells, where they soon grow into 

 small needles having latei-al outgrowths. The ectodermal 

 axial skeleton of the Goi-gonidse arises later than these 

 mesodermal parts of the skeleton. It must be regarded as 

 a cuticular secretion of the ectoderm of the basal foot-plate 

 (v. Koch), and at its first appeai'ance consists of a thin 

 yellowish pellicle, which may be compared to the sheath 

 of Cornularia and Clavularia. There is soon noticeable on 

 this basal plate a small prominence, which grows up into 

 a process composed of concentrically arranged corneous 

 lamelltB, and extends up between the mesenterial septa of the 



Fig. 34. — Section through the hody-wall of a young attached stage of Sym- 

 3)ocii«m coro (!o ides (after Kowalevskt et Maeion). cc, ectoderm ; eii, entoderm; 

 g, mesogloca ; ."ip, earliest fundament of the calcareous spicules in cells of the 

 developing mesoderm. 



primary polyp. Thus the ectoderm of the foot-plate must 

 be correspondingly invaginated, and thus it comes about 

 that the axial process of the ectoskeleton contained within 

 the polyp is covered by a continuous ectodermal lamella 

 (the axial epithelium), from which the further development 

 of this part of the skeleton takes place. In the further 

 course of development, during the progressive growth in 

 length, the young polyp and the axial skeleton do not take 

 the same direction ; the latter thereby acquires greater 

 independence, and represents the earliest fundament of the 

 whole axial skeleton which lies at the foiindation of the 

 entire colony subsequently produced by budding (Fig. 36 B) 

 (v. Koch, No. 86 i). 

 'On the other hand, Studer {Arch. f. Natiirg. Jahrg., 1887) has 



