66 WALTER. HE APE. 



The plate of cells in the Mole flattens out again, while in the 

 Field Mouse it becomes further involuted within the vesicle, 

 and the lower middle portion becomes the epiblast of the em- 

 bryo, while the lateral portions form the amnion (Fig. c). In 

 this manner the secondary cavity is surrounded by inner mass 

 and outer layer cells, and into this cavity the embryo projects. 



In the common House Mouse a layer of hypoblast is first 

 formed below the rounded inner mass ; next above the latter 

 the outer layer cells become thickened and involuted within the 

 vesicle, carrying with them the solid inner mass. 



A cavity is subsequently formed in the latter, and it elongates 

 until it nearly reaches the opposite pole of the vesicle, to which 

 it was originally placed. 



Thus the cells of the inner mass alone line the secondary 

 cavity in this case, and into it the developing embryo projects 

 (Fig. D). 



The Rat develops similarly to the House Mouse, a secondary 

 cavity forming in the inner mass after it is involuted. 



In the Guinea-pig, however (Fig. e), the solid inner mass 

 appears to become attached to the opposite pole of the ovum at 

 a very early stage in the development of the blastodermic vesi- 

 cle, and the outer layer does not become involuted, if observa- 

 tions made by Dr. Wilson and myself be trustworthy, until a 

 considerably later period. A secondary cavity is eventually 

 formed within the inner mass, and into it the embryo projects. 



Thus a complete series of conditions may be traced in these 

 various animals between the inverted and normal types. In 

 the Rabbit the solid inner mass flattens out and remains on 

 the surface of the vesicle ; in the Mole it is first formed into a 

 curved plate, which subsequently becomes flattened out and lies 

 on the circumference of the vesicle ; in the Field Mouse it 

 flattens out first on the surface and then becomes and remains 

 involuted ; in the House Mouse it becomes involuted before 

 becoming flattened ; and in the Guinea-pig the inner mass 

 remains attached at the opposite pole of the ovum before it 

 becomes a vesicle, and an involution of the outer layer seconda- 

 rily takes place. 



