200 Journal of Comparative Neurology and Psychology. 



caudal to the chiasma. His also distincly states that the front 

 end of the entoderm meets the brain at the Basilarleiste and 

 only in the frog does he figure this part of the entoderm 

 (Seesel's sac) as extending forward beyond the Basilarleiste. 

 Although in discussing a question in which phylogeny is of 

 such great importance we should probably not consider the con- 

 ditions in mammals as of primary significance when they differed 

 from other vertebrates, yet it is obvious that we have reached 

 no concordant results in various classes of vertebrates with refer- 

 ence to the criteria thus far applied. 



In order to fix the anterior end of the head in primitive 

 vertebrates it is necessary not only to locate the point of con- 

 tact of brain, ectoderm and entoderm, but to locate this point 

 in those vertebrates in ivhich the prac oral entoderm is most completely 

 presen'ed. 



The animals which best meet the requirements of our defi- 

 nition are the selachians. In these (119, 106, 107, 57, 96, 97, 

 17) the praeoral entoderm occupies a median position forming 

 a bridge between the anterior head cavity and somite i of the 

 two sides. It extends forward immediately beneath the brain 

 until it meets the ectoderm at the point of junction with the 

 brain plate. Laterally from this point are the ectodermal thick- 

 enings which constitute the nasal plates. These are connected 

 across the middle line by a thickened ridge which is continuous 

 with the border of the neural plate before, and for a short time 

 after, the rolling up and closure of the neural tube. Just below 

 this point appears the ectodermal pit which forms the hypophysis. 

 So far as selachians are concerned, the anterior end of the head 

 and brain can scarcely be put at any point on the brain wall which 

 is topographically below or behind this meeting place of entoderm 

 with brain plate and nasal plates — for all such points are behind 

 the anterior end of the entoderm. To seek the anterior end of the 

 brain at any point topographically in front of the lower border 

 of the neuropore implies that some part of the margin of the 

 neural plate may be morphologically anterior. Such a view 

 could be accepted only if it were shown that the neuropore 

 closed by fusion of its lips from below as well as from above. 



