18 A. A. W. HUBRECHT. 
This particularity may have passed on in the ancestral line of 
the chordata. 
Now, if in our further phylogenetic speculations concerning 
the Protetrapods and their descendants that live at the pre- 
sent time, we were to start from an oviparous aquatic animal, 
whose early developmental stages are provided with a larval 
envelope, we understand that, when any such animal came to 
adapt itself to inhabit the dry land it would doubtlessly score 
certain advantages if at the same time it became viviparous. 
Its adaptation would certainly be the more complete if, 
for its reproduction, it were independent of the aquatic 
medium. And towards the efficiency of this viviparous 
condition the larval envelope could immediately contribute 
by the mere change of its protective or locomotor 
significance into an adhesive one. ‘This again would be 
facilitated if the larval envelope, increasing in surface, were 
to develop into a spherical vesicle precociously forestalling 
the further development of the mother-cells of the embryo 
of which this larval envelope had originally been an organ 
of protection and of locomotion. Subsidiarily this vesicular 
shape would contribute towards the retention of the deve- 
loping egg for a longer time in the maternal genital ducts. 
And at the same time the possibility would arise of intro- 
ducing through the wall of this swollen blastocyst not only 
fluid to increase the swelling, but also nutritive matter to 
further the growth of the didermic ‘ Anlage” contained 
in it. 
All these circumstances accompanying the transition to an 
atmospheric environment would at the same time be unques- 
tionable advantages of protection and nutrition to the embryo, 
such as are already sporadically obtained in certain fishes 
(Mustelus, Zoarces, and others). Besides this, however, 
another advantage might ensue, viz., the possibility of this 
larval and transitory layer becoming vascularized in aid of a 
yet more thorough system of nourishment at the expense of 
the maternal circulatory system. 
And it is this what we actually observe in the mammals 
