HEAD ORGANIZER AND TRUNK ORGANIZER 89 



inheritance is not known, but it is thought that the nucleoproteins in the 

 cytoplasm are responsible for this kind of inheritance. Such a suggestion is 

 reasonable, since nucleoproteins in the nucleus appear to be associated with 

 nuclear inheritance. It is only logical to assume that cytoplasmic nucleo- 

 proteins are associated with cytoplasmic inheritance. 



Head organizer and trunk organizer 



Let us return to a question regarding the organizer itself. Is the organizer 

 divisible in the sense of having various distinguishable parts ? Does the organ- 

 izer show any regional differences within itself, or is it a homogeneous 

 region? A rather simple experiment shows that the organizer is not homo- 

 geneous. In the early neurula stage, when the neural plate has just formed, 

 the organizer is the roof of the archenteron. If we cut through the neural 

 plate in the head region, we expose the roof of the archenteron in the head 

 region, as shown in Figure 41. If we perform a similar operation in the trunk 

 region we expose the organizer underneath the trunk. Now we can test the 

 organizer ability of these two regions by placing them in contact with the 

 ectoderm of an early gastrula. We simply transfer the pieces of the roof of 

 the archenteron to an early gastrula in both cases and then follow the develop- 

 ment of these two gastrulae. 



Here are the results. The gastrula receiving the head end of the roof of 

 the archenteron forms a larva in which a second head develops from the 

 ectoderm of the ventral body region. This secondary head, like the normal 

 head of the host to which the organizer has been transplanted, will contain 

 a brain, eye, and nasal structures. The more posterior region of the organizer 

 has quite a different effect on the ectoderm. In this case from the ventral 

 region of the host we obtain a secondary trunk and tail. Internally, this 

 induced structure has the characteristic morphology of the trunk and tail. 

 Thus, we may call the anterior roof of the archenteron a head organizer and 

 the posterior roof of the archenteron a trunk organizer. 



The situation may be clarified somewhat by noting the relation of the 

 head and trunk organizers to the roof of the archenteron at the end of 

 gastrulation. At this time invagination of the chordamesoderm has taken 

 place. In Figure 42 the areas originally above the dorsal lip are numbered 1 

 through 5 and they now form the roof of the archenteron. Regions 1 and 2 

 are relatively anterior and are in the head region; regions 3, 4, and 5 are 



