322 THE RECAPITULATION THEORY 



between successive stages provided with antlers. 

 In both cases the explanation is afforded by con- 

 venience, whether of the embryo or adult. 



Many embryological modifications or distortions 

 may be attributed to mechanical causes, and may 

 fairly be considered under the head of develop- 

 mental conveniences. The amnion of higher 

 vertebrates is a case in point, and is probably 

 rightly explained as due in the first instance to 

 sinking or depression of the embryo into the yolk, 

 in order to avoid distortion through pressure 

 against a hard unyielding eggshell. A similar 

 device is employed, presumably for the same reason, 

 in the early development of many insect embryos ; 

 and the depression of the Taenia head within 

 the cyst is a phenomenon of very similar nature. 



Restriction of the space within which develop- 

 ment occurs often causes displacement or distortion 

 of organs whose growth, restricted in its normal 

 direction, takes place along the lines of least resist- 

 ance. The telescoping of the limbs and other 

 organs within the body of an insect larva is a simple 

 case of such distortion ; and a more complicated 

 example, closely comparable in many ways to the 

 invagination of the Taenia head, is afforded by the 

 remarkable inversion of the germinal layers in 

 Rodents, first described by Bischoff in the guinea 

 pig, and long believed to be peculiar to that animal, 

 but subsequently and simultaneously discovered by 

 three independent observers, Kupffer, Selenka, and 

 Eraser, to occur in varying degrees in rats, mice, 

 and in other rodents. 



