412 WALTER HEAPE. 



The Development of the Mole (Talpa Europea . 

 The Formation of the Germinal Layers, and 

 Early Development of the Medullary Groove 

 and Notochord. 



By 



Walter Heape, 



Demonstrator iu the Morphological Laboratory, Cambridge. 



With Plates XXVIII, XXIX, XXX, XXXI. 



In the following paper I propose to commence with a 

 description of the fully-segmented ovum, leaving the details 

 of the segmentation for a future communication j thence to 

 trace the growth of the blastodermic vesicle and the ultimate 

 formation of the hypoblast, epiblast, and mesoblast of the 

 embryo ; to follow the early stages of the development of the 

 medullary canal and notochord ; and finally to touch upon the 

 phenomenon of the inversion of the layers in certain mammals, 

 and to endeavour to show that in the mole there exists in 

 development an intermediate condition between the inverted 

 type, of which the guinea-pig is an example, and the normal 

 type as exemplified by the rabbit. 



Owing to the difficulty of keeping moles alive and the still 

 greater difficulty of observing their breeding habits Avhen in 

 captivity, I have found it impossible to determine the exact 

 age of any embryos, and am obliged to fix their relative age in 

 accordance with their size, and what appears to me to be the 

 course of their development. 



Under these circumstances it will be convenient to divide 



