32 EMBRYOLOGY 



the central cavity arise, in which the fundaments of an 

 exhalent canal system, lined with entoderm, are to be 

 recognized, so that in this manner we arrive by gradual 

 transitions at the distribution of the canals shown in Fig. 

 10 D, which serves as a plan for most sponges. 



This series, resulting from the comparison of different 

 genera, makes it probable that the ontogenetic origin of the 

 gastro-canal system by the formation of diverticula from a 

 single common central cavity, as it has been observed in 

 many forms, represents the original mode of development. 

 As to the development of the calcareous or siliceous spicules, 

 it appears to be certain that they are formed within skeleto- 

 genous mesoderm cells. The circumstance that the forms of 

 the spicules frequently merge into one another, and that in 

 many of the rod-like spicules an axial cross has been ob- 

 served in connection with the central canal, suggesting their 

 origin from triaxial spicules, gave rise to certain specula- 

 tions on the fundamental form of the spicules occurring in 

 the different groups and their derivation from the soft parts 

 of the body through simple mechanical conditions. Thus 

 F. E. ScHULZE (Ahh. kgl. Acad. Berlin, 1887) found that the 

 fundamental form of the regular triaxial spicules of the cal- 

 careous sponges was determined by the regular alternating 

 position of the pores in the wall of the original ascon-like 

 animal, and that the peculiar initial quadriradiate form of the 

 Tetractinellidae (as well as that of the sponges derived from 

 them, the Monactinellidse and the horn sponges) were explain- 

 able by the closely crowded position of the spherical ampullae 

 and the resulting forms of the soft parts of the body, whereas 

 in the Hexactinellidoe the arrangement of the trabeculas of the 

 soft parts led to the fundamental form of the regular sexi- 

 radiate spicules. 



Whereas the hard parts just mentioned develop within 

 cells, the horn fibres must be considered as cuticular secre- 

 tions, for, according to F. E. Schulze, they are deposited on 

 the inner surface of mesoderm cells (so-called spongioblasts) 

 arranged like an epithelium. 



Non-sexual Reproduction of Sponges. — In this con- 

 nection is to be mentioned the formation, by means of budding, 



