SOME PROBLEMS OF RBPRODUOTION. 587 



iong before boiling, aud then disappeared. The inference is 

 that a saccharoid substance is present in the blastoderm^ and 

 an inverting ferment effects its change. It would be most 

 desirable that some more delicate and satisfactory test for the 

 presence of minute quantities of proteoses and albuminoses 

 should be devised or discovered. My chief ground of con- 

 fidence in my positive results is derived from the failure to 

 obtain them under the circumstances of delay which I have 

 already stated. 



The noteworthy fact is that in the frog's eggs the digestion 

 must needs be purely intra-cellular^ and is prior to the 

 differentiation of a well-marked gastric layer. I think that 

 the same applies very largely to the blastoderm of the chick; 

 for I have failed to detect proteoses and peptones in the yolk 

 itself on the third and fourth day, while the cells of the blasto- 

 derm are themselves gorged with yolk granules, which it 

 seems likely they have absorbed, or rather swallowed amoeba- 

 fashion.^ 



A question that arises here, as in the holozoic Protista, is 

 the origin of the acid required for the peptic digestion and its 

 disposal. Concerning the origin and nature of the acid it is 

 hard to speak; but in the frog, as in the chick^ the early 

 formation of the alkaline blood gives a possible clue; or it 

 may be formed by dissociation. As regards the disposal of 

 any excess of acid, I had thought that the alkalinity of the 

 albumen might serve to neutralise the acidity of the digestive 

 juice ; but a comparison (using methyl orange as the indi- 

 cator) of the acidity of the albumen in three eggs that had 

 been incubated for three days with that of three eggs of the 

 same batch kept at the room temperature in this rather cool 

 May revealed no difference. 



One thing is clear as the result of this : all probability 



* The "intra-cellular digcstiou" of so many of the lower Metazoa must be 

 preceded by a tiiic partial stomacliic digestion that causes the food to fall 

 apart into small masses of a size suitable for the eudodcrm cells to swallow by 

 their pseudopodia. 



