CHAP. IX 



DEVELOPMENT 



227 



these types are but the extreme terms of a continuous series of 

 forms which have all the same essential constitution and undergo 

 the same metamorphosis. 



The amphiblastula of Sycon raphanus (Fig. Ill) consists of 

 an anterior half, formed of slender flagellated cells, and a posterior 

 half, of which the cells are large, non-flagellate, and rounded. 

 These two kinds of cell are arranged around a small internal 

 cavity which is largely filled up with amoebocytes. The 

 flagellated cells are invaginated into the dome of rounded cells 

 during metamorphosis, in fact, become the choanocytes or gastral 

 cells; the rounded cells, on the other 

 hand, become the dermal cells an 

 astonishing fact to any one acquainted 

 only with Metazoan larvae. 



A typical parenchymula is that of 

 Clathrina blanca (Fig. 112). When 

 hatched it consists of a wall surround- 

 ing a large central cavity and built up 

 of flagellated cells interrupted at the 

 hinder pole by two cells (p.g.c) the 

 mother -cells of archaeocytes. Before 

 the metamorphosis, certain of the flagel- 

 lated cells leave the wall and sink into 

 the central cavity, and undergoing Fig. 112. 

 certain changes establish an inner mass 

 of future dermal cells. By subsequent 

 metamorphosis the remaining flagellated 

 cells become internal, not this time by 



invagination, but by the included dermal cells breaking through 

 the wall of the larva, and forming themselves into a layer at the 

 outside. 



In the larva of G. blanca, after a period of free-swimming 

 existence, the same three elements are thus recognisable as in 

 that of Sycon at the time of hatching; in the newly hatched 

 larva of C. llanca, however, one set of elements, the dermal cells, 

 are not distinguishable. The difference, then, between the two 

 newly hatched larvae is due to the earlier cell differentiation 

 of the Sycon larva. 1 



Now consider the larva of Lcucosolenia. It is hatched as a 



Median longitudinal 

 section of parenchynmla 

 larva of Clathrina blanca. 

 p.g.c, Posterior granular 

 cells archaeocyte mother- 

 cells. (After Minchin.) 



1 Cf. Minchin in E. Ray Lankester's Treatise, p 



