THE MESODEEM AND NOTOCHOED. 25 



considerable time. At a later period the notochordal cells are excalated from the 

 entoderm, and then they form a cylindrical rod of cells which occupies the median 

 plane, lying between the floor of the ectodermal neural groove and the entodermal 

 roof of the primitive alimentary canal, which, in the meantime, has been more or 

 less moulded off from the dorsal part of the entodermal sac (Fig. 37). For a still 

 longer time the caudal end of the notochord remains connected with the anterior 

 end of the primitive streak, and its cephalic end is continuous with the entoderm 

 of a small portion of the embryonic area, which lies immediately in front of the 

 anterior end of the neural groove and which becomes bilaminar by the disappearance 

 of the primary mesoderm. This region, because it afterwards forms the boundary 

 membrane between the anterior end of the primitive entodermal canal and the 

 primitive buccal cavity or stomatodseum, is called the bucco-pharyngeal membrane 

 (Fig. 55, p. 42). It disappears about the third week of embryonic life, and immedi- 

 ately afterwards the anterior end of the notochord separates from the entoderm, 

 but the posterior end remains continuous with the primitive streak, until the for- 

 mation of the neural tube is completed. 



After a time the cylindrical notochordal rod is surrounded by secondary 

 mesoderm which becomes converted into the vertebral column of the adult. As 

 the vertebral column is formed the notochord is enlarged in the regions of the inter- 

 vertebral fibro-cartilages and for a time assumes a nodulated appearance (Fig. 60). 



Ultimately the notochord disappears, as a distinct structure, but remnants of 

 it are believed to exist as the pulpy centres of the intervertebral fibro-cartilages. 

 The extension of the notochord into the region of the head is of interest from a 

 morphological, and possibly also from a practical point of view. It extends 

 through the base of the cranium from the anterior border of the foramen magnum 

 into the posterior part of the body of the sphenoid bone. Its presence in the 

 posterior part of the skull suggests that that region was, primitively, of vertebral 

 nature. As the notochord passes through the occipital portion of the skull it 

 pierces the basilar portion of the occipital region first from within outwards and then 

 in the reverse direction. It lies, therefore, for a short distance, on the ventral 

 surface of the rudiment of the occipital bone, in the dorsal wall of the pharynx, 

 and it is possible that some of the tumours which form in the dorsal wall of the 

 pharynx are due to the proliferation of remnants of its pharyngeal portion. 



The Differentiation of the Secondary Mesoderm. It has already been 

 noted that a portion of the inner mass of the human zygote becomes converted 

 directly into mesoderm which may be called, for convenience, primary mesoderm. 

 It was stated also that the wall of the larger of the two inner vesicles of the 

 zygote consists of ecto-mesoderm, that term being intended to convey the idea 

 that the cells of the wall of the larger inner vesicle were the progenitors of both 

 ectodermal and mesodermal cells. 



As soon as the larger of the two inner vesicles is formed two areas of its 

 wall are defined : (1) the part in contact with the smaller inner or entodermal 

 vesicle and (2) the remainder. As future events prove, the cells of the larger 

 area, which is not in contact with the entodermal vesicle, simply produce 

 ectodermal descendants which line the inner surface of a sac-like covering of the 

 embryo termed the amnion ; they are, therefore, the predecessors of the amniotic 

 ectoderm. 



The cells of the larger inner vesicle, which lie adjacent to the smaller entoderm 

 vesicle, and are merely separated from the entoderm by a thin layer of primary 

 mesoderm, take part in the formation of the embryo ; forming, with the entoderm, 

 the embryonic area from which the embryo is evolved. These cells are the fore- 

 runners of both ectoderm and mesoderm, and as the mesoderm developed from 

 them is differentiated after the formation of the primary mesoderm it may be 

 termed secondary mesoderm or primitive streak mesoderm ; the latter term being 

 applied because it is differentiated in a linear region called the primitive streak 

 (p. 23). It is the formation and fate of this primitive streak mesoderm which is 

 now to be considered. 



At first the embryonic area is circular in outline, at a later period it becomes 

 ovoid, and in the narrower or caudal portion of the ovoid area a linear thickening 



