478 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



upgrowth round the body of the embryo — the amniotic fold — would 

 now become purely somatopleural (Fig. 216, C). 



(9) As soon as the amniotic fold extended so far over the body 

 of the embryo as to roof it in completely it would at once assume a 

 new importance in protecting the delicate body of the embryo, 

 enclosed within it as in a water jacket, from the dangerous jars and 

 shocks incidental to a terrestrial existence. In correlation with the 

 importance of this function of the closed amnion we might expect to 

 find a tendency for its closure to be accelerated. As a matter of fact 

 it will be found that in various mammals, includ- 

 ing man, the amniotic cavity is closed from the 

 beginning. 



IV. Viviparity in the Lower Vertebrates. 

 — In many different groups of animals the em- 

 bryonic phase of development is passed within 

 the oviduct (uterus) of the mother. The advan- 

 tages of this are obvious, for not only is the 

 young individual sheltered to a great extenffrom 

 the struggle for existence, as it is even within 

 an egg-shell, but it forms for the time being as it 

 were part of the body of a complete adult 

 individual with its full equipment for holding its 

 own in the struggle. It is in the group Mam- 

 malia amongst Vertebrates that viviparity reaches 

 its highest development, as the final touch in 

 their adaptation to a terrestrial existence, but 

 it is of interest to notice that the phenomenon 

 occurs, in a less highly elaborated form, here and 

 there amongst the lower Vertebrates — Fishes, 

 Amphibians, and Reptiles. 



Thus among the Elasmobranch fishes x there 

 are numerous genera in which the early stages 

 in development are passed through in the uterus. 

 In such cases we find in the first place a well-marked tendency 

 towards the reduction of the protective egg-envelopes which are 

 no longer necessary. Thus there is found, as a rule, during early 

 stages a typical set of egg-envelopes, but the horny shell is very thin 

 and weak as compared with that of oviparous Elasmobranchs and as 

 development goes on (embryo of 7-8 cm. in Acanthias) it becomes still 

 thinner, breaks up, and disappears. 



A curious feature in such cases is the tendency for a group of 

 eggs to be enclosed in a common set of envelopes instead of each egg 

 having its own set. Thus in Acanthias (Fig. 217) there are commonly 

 from two to six eggs enclosed in a common shell ; in Trygonorhina 

 two or three, in Ehindbatus seven or eight. 



The embryo within the uterus is still nourished primarily by the 

 yolk in its yolk-sac. This primitive mode of nourishment has not 



1 See Gudger, 1912. 



Pig. 217.— Egg-shell of 

 Acanthias enclosing 

 two eggs. (The divi- 

 sions of the scale re- 

 present millimetres. ) 



