410 



just older than Balfour's stage B, it is quite clear that segmentation 

 is expressed in the outer germ-layer, before the mesoblast of the 

 same region is so divided, and the epiblastic segments must be in- 

 dependent of any formative influence of the latter. A mistaken idea 

 has also been conveyed through the assumption that the neural seg- 

 ments arise after the neural tube is established. Minot, in expressing 

 his conception of the formation of the neural segments, based upon 

 the descriptions of Orr and Mc Clure, says: "Their appearance seems 

 to depend upon the development of the primitive segments of the 

 mesothelium. When the segments are fully formed, and before their 

 inner wall has changed into mesenchymal tissue, they press against 

 the medullary tube and oppose its enlargement; at least one sees 

 that the tube becomes slightly constricted between each pair of 

 segments and slightly enlarged opposite each intersegmental space" 1 ). 

 But here, again, the fact that the neural segments arise long before 

 there is any neural tube will render this view no longer tenable. 

 The question might arise here is not this very early segmentation 

 evanescent in character, or irregular in occurrence, and is not the 

 later epiblastic segmentation, present in conjunction with mesoblastic 

 segmentation, another affair? In answer to this question, I must say 

 that I have uniformly found this division into metameres in the 

 earliest stages, in all specimens I have examined, it is, therefore, not 

 irregular in occurrence. My material has been prepared in a number 

 of different reagents: chromic acid, chrome acetic (Davtdoff's fluid), 

 picro-sulphuric, picro-nitric, and Flemming's stronger solution, but, 

 whatever the treatment, the segments in these early stages have been 

 alike in number, and similar in appearance on both sides of the 

 neural plate. I have also traced the earliest formed segments directly 

 into the later stages and, therefore, they are not evanescent. 



Head and Trunk. 

 It would be a great convenience to anatomists to have some 

 means of distinguishing between the head and trunk of very young 

 embryos. It is generally regarded as impossible, on account of the 

 lack of definite landmarks, to assign such a line of division, in early 

 stages, before the origin of the auditory vesicle. As Sedgwick says, 

 in the same article referred to above: "The term head here must be 

 regarded as meaning the anterior end of the body, for it is not 

 possible in these young embryos to distinguish the head from the 



1) Human Embryology, New York, 1892, p. 604. 



