450 GASTRULATION 



forming areas begins. The organization of the blastoderm (blastula) is such 

 that presumptive pre-chordal plate mesoderm and notochordal areas lie far 

 anteriorly toward the midcentral part of the epiblast. In other words, a con- 

 tiguous relationship between presumptive pre-chordal entoderm (i.e., ante- 

 rior foregut entoderm) and presumptive pre-chordal mesoderm and the pre- 

 sumptive notochord at the caudal margin of the blastula does not exist. 

 Consequently, a different procedure is utilized in bringing the foregut ento- 

 derm, pre-chordal mesoderm, and anterior notochord together. That is, the 

 head-organizer materials must be assembled together in one area underneath 

 the cephalic portion of the neural plate. This is accomplished by two methods: 



( 1 ) The use of a large invaginative process, the notochordal canal, which 

 projects pre-chordal plate mesoderm and notochord cranio-ventrad 

 toward the foregut entoderm in the hypoblast below, as described in 

 figure 200 relative to the reptiles or in figure 207B of the human 

 embryo and 



(2) the use of another and less dramatic method for getting the head- 

 organizer materials together, the vestigial invaginative process which 

 produces the primitive pit and Hensen's nodal area. 



The latter mechanism succeeds in getting pre-chordal plate mesoderm and 

 notochord down between the epiblast and hypoblast and forward to unite with 

 the anterior part of the foregut entoderm. (See Adelmann, '22, '26; Pasteels, 

 '37b.) 



It is not clear whether the invaginative behavior which produces the primi- 

 tive pit or notochordal canal is an autonomous affair or whether it may be 

 dependent upon the inductive activities of the entoderm below. More experi- 

 mentation is necessary to decide this matter. The work of Waddington ('33), 

 however, leads one to conjecture that inductive activities may be responsible. 



Regardless of the factors involved, cephalogenesis or the genesis of the 

 head is dependent upon the assemblage of anterior foregut, pre-chordal meso- 

 derm, and anterior notochordal cells beneath the cephalic portion of the 

 neural plate as described on page 401. 



K. Blastoporal and Primitive-streak Comparisons 



From the considerations set forth above, it is clear that the area of the noto- 

 chordal canal or primitive pit (i.e., Hensen's nodal area) corresponds to the 

 general region of the dorsal lip of the blastopore of lower vertebrates, whereas 

 the dorso-lateral and lateral lips of the blastopore of lower forms correspond 

 to the body of the primitive streak in higher vertebrates (Adelmann, '32). 



