A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 1 '.> 1 



inclined to carry the comparison still farther, the peculiar features of the primitive 

 groove mentioned on page 121 might be interpreted as an attempt at the formation 

 of lateral blastoporic lips. How much of the mesodenn of the embryo appears first 

 as strictly peristomal we of course can not say. 



The consideration of the head process involves also the tangled question of 

 tin' entodcrm. The head process of the primitive streak (Kolliker), 1'ebauche de 

 1'archenteron of Van Beneden, Bonnet's Urdarmstrang, or the Mesodermsackchen 

 of O. Hertwig, is one of the most important features of the area embryonalis. In 

 its formation, and that of the primitive streak, we have the essentials of gastrulation 

 in man; in the cavity of the head process, the archenteric canal (Urdarmkanal), is 

 retained all that is left of the cavity of the primitive gastrula, the archenteron. 

 From this head process are derived, to what extent it is impossible to say, gastral 

 mesoderm, further chorda, for the most part, and (for aught we know) more or less 

 of the entoderm of the gut-tract. From the foregoing it will be clear that we do not 

 agree with Keibel (1910, 1913) and Hubrecht (1905, 1909) in considering the forma- 

 tion of the two-layered stage, ectoderm and entoderm, as constituting the process of 

 gastrulation. That entoderm formed by delamination is essentially secondary or 

 yolk entoderm, the paraderm of von Kupffer, Wenckebach's csenogenetic entoderm, 

 the lecithophor of van Beneden. 



To what extent this first-formed layer is concerned in the formation of the diges- 

 tive tract we do not know; certainly in some forms its role is by no means an 

 exclusive one. The fact that this yolk entoderm fuses with the head process but not 

 with the primitive streak is but evidence as to its csenogenetic character. The only 

 support of the views of Keibel and Hubrecht is the supposition that this secondary 

 entoderm is the sole and only source of the gut entoderm. The theory and the 

 entoderm stand or fall together. In the walls of the head process, i. e., bounding the 

 archenteric canal, we would expect to find primary or protentoderm, Bonnet's 

 Urentoderm, the palingenetic entoderm of Wenckebach. If the lumen of the head 

 process is in reality an archenteric canal, then we would expect it to give rise to 

 mesoderm (segmented), chorda, and gut entoderm and such, with the reservations 

 given above, seems to be the case. If the head process is simply the anlage of the 

 chorda plus some mesoderm (whence the misnomers chorda! or notochordal canal, 

 chordulation, etc.), why should it contain a definite although inconstant canal com- 

 municating with the exterior; why so much more material than is required for the 

 chorda, and why its fusion and communication with the yolk-sac? The answer is 

 that in the formation of the head process and not in the delamination of the second- 

 ary entoderm we have a process which can be designated as gastrulation. 



As concerns the derivatives of the head process, the case of the chorda is perfectly 

 clear. At this stage its anlage is contained in the dorsal, epithelial wall of the canal, 

 the notochordal plate. The fate of the ventral wall or floor of the canal, the plaque 

 enterique of van Beneden, is uncertain. It fuses early with the yolk entoderm or 

 lecithophor immediately beneath to form the plaque lecithoenterique. The loss of 

 the floor, from the rearrangement of its cells, results in the confluence of the archen- 

 teric canal and the cavity of the yolk-sac. This process is naturally csenogenetic. 



