no GJ<1NERAL CONSIDERATIONS. 



for does he not say, aud does not Metschnikoff quote him as 

 saying (No. 39, p. 156), that he thought it probable that the 

 ancestors of Coelenterates possessed a solid endoderm of 

 amoeboid cells ? 



This is not the only point in which Metschnikoff has mis- 

 understood Balfour's views. On p. 141 of No. 39, he ascribes 

 to him the view that an amphiblastula form would represent 

 more nearly than any other the transition between the Pro- 

 tozoa and Metazoa. Balfour maintained no such position, as 

 has been already pointed out by the translator of Metschnikoff's 

 paper on the " Intracellular Digestion of Invertebrates," 

 ' Quarterly Journal of Microscopical Science/ vol. xxiv, p. 107, 

 note. 



But to resume : Metschnikoif supposes that the parenchy- 

 matous ancestor was preceded by a hollow spherical form, the 

 blastula., the cells of which were all alike ; and that the blastula 

 became a parenchymatous gastrula by the migration inwards 

 of cells from its external wall. 



I do not understand on what grounds Metschnikoff is so 

 strongly disposed towards the view that the hollow blastula 

 represents a primitive form. 



There aix many cases, even amongst the Coelenterates, in 

 which a hollow blastula is not formed and segmentation gives 

 rise to a solid planula; and in the rest of the animal kingdom, 

 the cases, in which segmentation gives rise to a solid embryo 

 are quite as numerous, if not more so, than those in which the 

 reverse holds. 



I would even go further than this, and maintain that 

 Metschnikoff's view, that the ectoderm is primitive and the 

 endoderm secondary — arising from the former by inwandering 

 — is not more in accordance with the facts of embryology than 

 the opposite view, viz. that the endoderm is primary, giving 

 rise to the ectoderm by budding-off cells — outwandering as it 

 may be called. In almost all Invertebi'ate groups there are 

 instances of the latter process, in which the ovum, before or 

 after division into two or more large cells, buds out a number 

 of small cells which form the ectoderm ; either itself, or the 



