THIRD PERIOD OF DEVELOPMENT. 75 



body which are not situated in the longitudinal axis 

 describe a spiral 1 line in their forward movement. 



The embryos maintain this peculiar movement for 

 still quite a long time and do not give it up till the 

 body has assumed a very extended fish-like form. 



The eggs which have attained to their development 

 in the glasses lie on the bottom of the latter. After 

 leaving the egg membrane the embryos make their 

 way to the surface of the water. 



In the next stages, in which the formation of the 

 third mesoblastic somite takes place, there can as yet 

 be observed in the living object only a sharper optical 

 appearance of the mesoblastic somites (Fig. 41). 



So far as concerns the alteration of the outer bodily 

 form, during the formation of the three first meso- 

 blastic somites a continual elongation of the embryo 

 may be observed. The dorsal surface too, which also 

 after the enclosure of the neural plate was further 

 deepened, becomes gradually flat, and at the same time 

 the cavity of the neural canal underneath the super- 

 ficial epithelial la}'er becomes visible. 



1 A similar spiral movement may also be observed in other 

 bilaterally formed larvse (Echinoderm larvse, Worm larvse, etc.), 

 as was lately pointed out by Metschnikoff. 



