180 PRIMITIVE FORMS OF LIFE 



sufficient in number to become differentiated and to constitute 

 the essential parts of the embryo. As the nuclear substance and 

 the vital plasma of the egg were not yet exhausted, the cells 

 formed last remained outside the embryo, so to speak, and 

 merely served to help in nutrition. The egg itself was formed 

 in the same way ; the oldest elements within the ovary 

 or those best adapted for nourishing themselves alone 

 developed and nourished themselves at the expense of the 

 others. 



The embryo naturally took its food from the region 

 immediately beneath it, the vitellus or yolk. It gradually 

 assimilated its substance, and consequently sank down into 

 it. Thereafter the extra embryonic blastoderm gradually spread 

 up and closed over the embryo, whilst it contracted below, thus 

 forming an envelope which is the less mysterious in origin in- 

 asmuch as it also forms for identical reasons and in the same way 

 around the embryos of insects. This is the amnion. The sac 

 becomes filled with liquid, and henceforth the embryo is 

 protected against all risk of desiccation. Respiration is accom- 

 plished in a fashion that is somewhat roundabout, but 

 therefore all the more interesting. Minute renal ducts early 

 appear in the mesoderm and empty the products of their 

 secretion into a pocket or receptacle which is nothing more 

 nor less than the embryonic bladder. This sac has no 

 external opening. Hence it retains the products of secretion 

 which it receives, and becomes so greatly distended as to 

 nearly line the amnion itself, while a close network of vessels 

 is formed in the thickness of its walls ; this is the allantois. 1 

 As this membrane is extremely rich in vessels, is separated only 

 from the outer air by the thin containing membrane of the 

 amnion, and presents a large surface, it is admirably fitted to 

 assure respiration for the embryo all the time that the lungs are 

 not in communication with the outer air. Here we have a new 

 example of those changes of function resulting from the 

 accidental concurrence of circumstances to which Dohrn, as 

 we have already pointed out, so justly called attention. 

 Protected against desiccation, abundantly supplied with 

 nourishment, and breathing adequately, the embryo is now 

 developing under favourable conditions, and the process is 



1 From a\\as sausage, so-called because at first it has the form of a 

 closed tube recalling that of a sausage. 



