48 



Growth in length of the Vertebrate Embryo 



opening but there is a difference and this is a difference of 

 degree. In the Reptile the accumulation is slower. 



If we compare the stages of Mammal, Bird and Reptile at the 

 moment when the deuterogenetic centre becomes evident we find 



fl 



Fig. 26. 1. Hypogeophis after Brauer, 1907. 2. Platydactylus after Will, 

 1892, showing little fluid. 3. Vespertilio after van Beneden, 1912, showing 

 much fluid, fl fluid. 



very marked differences. In the Mammal the blastocyst cavity 

 is already large and is rapidly growing by continued increase of 

 hydrodynamical pressure producing tension in the layers above. 

 In the Bird the same condition is almost as evident. It is only 

 masked by the presence of the yolk mass, Fig. 28. 



