THE DEVELOPMENT OF THE MULE. 109 



posterior limb grew over the tail at an earlier period (Stage f, 

 fi^. 4). 



The anterior fold of the amnion (vide p. 128) is the so-called 

 pro-amnion of Beneden and Julin (No. 2). It must be noted 

 that up to the close of Stage j no signs of a folding off of the 

 tail end of the embryo can be observed, and, indeed it is not 

 until considerably later that this process takes place. 



The first junction of the two tubes to form the heart takes 

 place during Stage h, and is shown in fig. 8; while the side 

 view of the head of the embryo drawn in fig. 10 (Stage j) 

 shows the relation of the heart to the visceral arches, and the 

 arrangement of the latter. 



There are at this stage five visceral arches. Faint grooves 

 indicating the partial formation of two and even three visceral 

 arches may be discerned during Stage h, but it is not until 

 Stage J is reached that they can be satisfactorily outlined. 



For the comparison of these figures with figures of other 

 mammalian embryos I would refer to papers Nos. 3, 4, 5, 6, 7, 

 9, and 10. 



The Epiblast. 



Soon after the epiblast is first definitely produced it is in the 

 form of a plate of columnar cells of uniform thickness over the 

 whole embryonic area, and passing abruptly at the edge into 

 the flattened epiblast cells which cover the remainder of the 

 embryonic vesicle. This stage is figured in a former paper, 

 No. 8, fig. 30. 



During the primitive streak stage of growth and the early 

 formation of the medullary groove, the lateral epiblast becomes 

 reduced in thickness and at the edge of the area the cells 

 gradually assume a flattened condition and blend without 

 a break with those of the vesicle (1. c. figs. 32 — 36, and 

 43—46). 



The appearance of protovertebrae and the deepening of the 

 medullary groove is attended by a further modification of the 

 epiblast of the embryo. 



During Stages e to g the median portion becomes thickened 



