180 A. A. W. HUBRECHT, 



cell-material, which is going to be the new worm, is also very 

 considerable, to remove the objection that in this respect 

 mammals would stand isolated. And we may go one step 

 further and say that it is easy to nndei-stand wiiy this con- 

 siderable extension of the outer larval layer has come into 

 existence. When we look back along the line of phylogetietic 

 descent we can imagine that at the period when, for the first 

 time, aquatic animals became inhabitants of the land, four- 

 footed instead of four-finned, and adapted for aerial breathing 

 in addition to their respiration by the aid of gills, it may have 

 been a gi-eab advantage to them to become viviparous at the 

 same time, i.e. to keep their developing eggs inside of them, 

 Avhere they are better protected and can be better nourished 

 than outside of the mother. The atmosphere and the dry 

 land offer less favourable conditions for the development of 

 that small amount of protoplasm that forms the primordium of 

 each new being than does the water, and so viviparity is likely 

 to have been a parallel phenomenon to the exchange of the 

 aquatic for the terrestrial existence. 



We can see clearly that once an embryonic envelope, one 

 cell-layer thick, being present (on our original assumption, as 

 far back as the invertebrate ancestor), that this one-layered 

 larval envelope could obtain high efficiency for the incipient 

 viviparity if only it bulged out as much as possible, thereby — 



(1) Preventing the egg from passing through the genital 

 ducts rapidly and being deposited, so to say, accidentally. 



(2) Enabling the egg to adhere in various ways to the 

 maternal tissues, either as a simple mechanical improvement 

 of what was attained (1), or at the same time inducing phago- 

 cytotic attacks on that maternal tissue. 



(3) Creating the occasion for individual trophoblast cells 

 of this outer layer to absorb fluids either from the uterine 

 cavity or accessory to the phagocytic processes alluded to 

 under (2), and thus accumulating nutritive material inside the 

 blastocyst. 



Furthermore, it is equally clear that, once the viviparity 

 having been establised, and the surface extension of the 



