398 NILS HOLMGREN 
secondarily modified. How the primary cyclostome forebrain 
originally was built up, it is not possible to make out in detail, 
but that the ventricular system of it was wider, with normal 
lateral ventricles and a sulcus limitans externus as in other brains, 
issure. The general form of such a forebrain may have been more 
or less like that of selachians with the bulbi olfactori situated lateral 
at the rostral part of the forebrain vesicle, as in an early embryo. 
The hemispheres were surely not joined in the dorsal mid-line, 
where the brain roof consisted of a tela, more or less as in Chi- 
maera. It is a priori probable that the transformation of the ex- 
terior of the forebrain was accompanied by transformations in 
the finer structure, but the nature of these transformations it is 
not possible to determine. In a young Ammocoetes, however, 
it is clearly seen that there are two pallial cell-layers present, a 
peripheral and a ventricular. These layers are in the medial 
part distinctly separated by a relatively cell-free space. These 
two layers are also to be distinguished in adult animals, but the 
cell-free space here is absent. Perhaps the conclusion from this 
fact may be admitted that there were two cortex layers present in 
the ancestral cyclostome, as in selachians, Dipnoi, etc., as will 
be shown later on. 
Myxinords 
The conditions in the forebrain of Petromyzon are repeated 
in Myxine, as I have already pointed out in my memoir on the 
remarkable brain of this animal (Svenska Vetenskapsakad:s 
Handlingar, vol. 60, no. 7, 1919). Through the reduction of the 
ventricle system and the hyperinversion of the pallial parts, the 
curious aspect characterizing this brain is realized. Here I may 
only call attention to the fact that the pallium contains a very 
distinct cortical layer. No zona limitans is present and it is diffi- 
cult to limit the pallium from the subpallial parts, the cells of 
both being equal in size and the basal cortical cells flowing over 
into the subpallial. In the subpallial parts no separate ganglia 
are to be recognized, as in Petromyzon. 
For particulars reference may be made to the above-quoted 
paper; here it is enough to accentuate that the principles of the 
brain construction are the same in Myxine and Petromyzon. 
