48 FORMATION OF NEURAL CANAL 



formation of the primitive streak and of the notochord, although involving invagination 

 phenomena, is not to be reckoned as part of the gastrulation process, but represents a 

 phylogenetic stage by which a radial ccelenterate form was converted into a proto-vertebrate 

 by the elongation of the gastrula and the formation of a dorsal plate which became the hoto- 

 chord. The primitive streak is thus not the gastrula-mouth of ontogeny, but represents the 

 protostoma of an Actinia-like form, as suggested by Sedgwick and Van Beneden. 



Assheton's * theory also involves the acceptance of two ontogenetic phases : a first (which 

 may be exemplified by the earliest phases in Tarsius) resulting in the formation of the forepart 

 of the head, and a second represented by the formation of trunk and tail. The idea is that the 

 lips of the circular blastopore grow actively so as to produce a cylindrical gastrula. The dorsal 

 lip, however, grows more actively in vertebrates, and produces the back and ultimately the tail 

 or post-anal part of the axis. The anus represents the blastopore, while the mouth is a new 

 opening (like the gill-slits) into the alimentary canal. Such a process is greatly modified, of course, 

 in the Amniota, whether those with mesoblastic eggs or mammals, and the ventral lip of the 

 gastrula is greatly masked by the presence of the yolk-sac. According to this conception, the 

 primitive streak is only a phase in the development of the embryonic axis out of the growing 

 blastema of the blastopore-lip, or secondary ' growth- centre.' 



EARLY CHANGES IN THE BLASTODERM, RESULTING IN THE 

 FORMATION OF THE EMBRYO. 



FORMATION OF THE NEURAL CANAL, NOTOCHORD, AND 

 MESODERMIC SEGMENTS. 



Neural canal. While the embryonic axis is developing, as described 

 above, a shallow groove appears on its surface in front of the primitive streak 

 (fig. 72). This elongates with the axis, and encloses, behind, the anterior end of 

 the streak with its neurenteric passage. Anteriorly and laterally it is bounded by 

 a fold of the ectoderm, the groove indeed being produced by the upgrowth of 

 the limiting folds (figs. 73 and 76). The thickened ectoderm of the groove is called 

 the neural plate, because the central nervous system is formed from it, and the 

 bounding folds are termed the neural folds. 



By the continued upgrowth of the neural folds (fig. 73) the neural groove is 

 converted into a deep furrow, and ultimately, by their fusion in the mid-axial line, 

 into a closed canal (fig. 82, p. 57). The neural plate is then separated from the 

 surface-ectoderm, and the closed canal becomes isolated as the rudiment of the 

 cerebrospinal axis. The closure of the canal appears in the human embryo to 

 begin in the region of the future trunk of the embryo, and proceeds forwards 

 and backwards. The point where the final closure occurs in front is called the 

 anterior neuropore. When the neural canal closes posteriorly, the neurenteric canal 

 comes to lie in its floor, but it is obliterated at an early stage by the fusion of 

 its lips and soon completely disappears. The anterior end of the neural canal 

 extends beyond the notochord, and becomes enlarged to form the anterior of 

 three primary cerebral vesicles round which the brain is formed. 



At the point where the lips of the neural folds meet, a mass of ectoderm-cells 

 forms a thickening known as the neural crest, from which, by a series of changes 

 afterwards to be described, the nerve-ganglia are formed. 



Notochord. It will be recollected that in last section we considered 

 the development of a plate of cells which we named the notochord-plate. We 



1 Anat. Anzeiger, xxvii. 1905. 



2 The literature of the germinal layers in mammals and man up to 1902 will be found fully given in 

 Hertwig, i. Part I. pp. 81 and 949. For a critical review of the earlier literature, see Born in Merkel 

 and Bonnet's Ergebnisse d. Anat. u. Entwickelungsgesch. i. 1891 ; and of the later literature, Keibel in 

 the same publication, x. 1901. 





