968 
Camerostoma and olfactory nerves 
Flabellum and nerve 
REPORT—1896. 
Olfactory organ and Ist nerve. 
Auditory organ and VIIIth nerve. 
Epimeral nerves to surface of pro- 
soma and mesosoma > 
Internal and External Skeleton. 
Internal skeleton. 
Branchial cartilages . 
Entapophysial cartilaginous 
ligaments 
Fibro-massive tissue (fore- 
runner of cartilage or 
‘ Vorknorpel’). 
External skeleton. 
Chitinous layer 
Sensory part of Vth nerve. 
Branchial cartilages. 
Subchordal cartilaginous ligaments. 
Muco-cartilage or ‘ Vorknorpel.’ 
Cuticular layer on surface of body and 
subepithelial laminated layer. 
Excretory Organs and Calomic 
Cavities. 
Coxal gland . 
Ist head cavity, preoral 
2nd head cavity. Cavity of pro- 
somatic segments 
Cavities to each mesosomatic 
segment. : . 
Heart and Vascular System. 
Dorsal heart . ; 
Longitudinal venous sinuses | ‘ 
Lacunar blood spaces of ap- 
pendages . . : ‘ 
Pituitary gland. 
Ist head cavity, proral. 
2nd head cavity, mandibular. 
Cavities of hyoid and branchial segments. 
Column of fatty tissue dorsal to spinal cord. 
Heart and ventral aorte. 
Lacunar blood spaces in velar and 
branchial appendages. 
The Possible Meaning of the Notochord. 
Although we can say that every structure and organ in the prosomatic and 
mesosomatic regions of Limulus, &c., is to be found in the head region of Ammo- 
coetes, we cannot assert the reverse proposition, that every organ in the head region 
of Ammocecetes is to be found in Limulus, &c., for we find a notable exception in 
the case of the notochord, a structure which is par excellence a vertebrate structure, 
and has in consequence given the current name to the group. Such a structure is 
clearly not to be found in Limulus and its allies; it has evidently arisen in connec- 
tion with the formation of the vertebrate alimentary canal from the oral and 
branchial chambers, and it evidently at one time possessed a functional significance, 
for the lower we descend in the vertebrate scale the more conspicuous it becomes. 
Unfortunately we know nothing of the condition of the notochord in the early 
extinct fishes, so that we are reduced to the embryological method of enquiry in our 
endeavours to find out the meaning of this organ. This method appears to point 
to the origin of the notochord from a tube connected with the alimentary canal, 
originally “therefore an accessory digestive tube; the reasons why such a view has 
been put forward are, first, the origin of the notochord from hypoblast ; secondly, 
the evidence that it is to a certain extent tubular; and thirdly, that it is an 
unsegmented tube extending from the oral to the anal regions of the body. 
Another argument, to my mind stronger than any other, is based on the principle 
that nature repeats herself, and if, therefore, we find the same proliferation of 
cells in the same place forming a series of solid notochordal rods, we may fairly 
argue that we are observing a series of repetitions of the same process for the same 
object. Now the formation of the head region of Petromyzon shows that at first 
a median proliferation of hypoblastic cells occurs to form the notochord, which 
then separates off from the hypoblast; later on a similar proliferation takes place 
to form the subnotochordal rod, which similarly separates off from the hypoblast ; 
later still, at the time of transformation, a third median proliferation of the cells of 
the hypoblast takes place, to form a solid rod of cells. This solid rod then com- 
mences to hollow out at the end nearest the intestine, and the hollowing out 
