434 EMBRYOLOGY 



mesencbyma cells at the sides of the storaach. The com- 

 prehension of this process is rendered more difficult by the 

 fact that the ambulacral and antambulacral surfaces are not 

 parallel, but nearly at right angles to each other. Between 

 the two lies the capacious stomach. In Fig. 209, which, 

 however, corresponds to a somewhat earlier stage, the water- 

 vascular rosette (H) is seen to be partly covered by the 

 stomach, whereas this in turn is partly covered by the funda- 

 ment of the antambulacral surface. The latter develops 

 further in such a way that from the calcareous concretions 

 a number of plates are formed (comp. infra), which cover 

 a pentagonal area. This grows out then into five processes, 

 thus establishing the dorsal surface of the arms, upon which 

 there appear wart-like elevations, from which the spines 

 arise later. 



At this stage the starfish already approaches the shape of 

 the adult animal, at least as far as regards its dorsal external 

 surface, and is seen attached to the larva, the posterior 

 end of which it has quite absorbed (Fig. 210). Its anterior 

 portion is still well preserved. Now, however, degeneration 

 also begins here. It gradually shrinks, its substance being 

 consumed by the phagocytic mesenchyma cells, undergoing 

 intracellular digestion, and being doubtless employed in the 

 formation of the new body (Metschnikoff, No. 40). At the 

 same time with these processes the size of the stomach 

 decreases, as a result of which the two surfaces of the star- 

 fish, which were separately developed, are able to approach 

 each other. They cover each other, and finally fuse. The 

 hitherto unclosed water- vascular rosette grows around the 

 oesophagus, and its radii elongate to form the ambulacral 

 vessels, which in their turn give rise to the feet. In this 

 process the distal end of the vascular fundament becomes 

 the so-called tentacle, but the feet are established laterally 

 in pairs. The youngest feet are always found next to the 

 tentacle, therefore at the tip of the arm, whereas the oldest 

 are crowded toward its base. The eye makes its appearance 

 as an accumulation of red pigment at the base of the tentacle 

 at a very early period. 



Even before this, there have been produced on the antam- 



