THE ENDOSKELETON 



685 



ventral region, it forms what is called a heterocercal tail, while if there 

 is the same upward bend of the spinal column but the ventral and dor- 

 sal fin-portions of the tail become alike as to size and shape, the tail is 



Fig. 405. 



Diagrams of the principal forms of tails in fishes. A, proto- 

 cercal fin (as in polypterus) ; B, heterocercal (as in sharks) ; C, homo- 

 cereal (as in most teleosts) ; D, homocercal (as in Amia). (After 

 Folsom. ) 



said to be homocercal. Homocercal tails are brought about by the neural 

 arches becoming smaller and the haemal arches becoming larger. 



THE SKULL 



Bone either forms in cartilage or membrane and it is quite common 

 to hear biologists speak of cartilaginous and membranous bone. How- 

 ever, recent investigations lead us to believe that the so-called membrane 

 is nothing more or less than cartilage drawn out very thin in those parts 

 where the greatest pressure is produced. This can be understood the 

 better if Figure 406 be carefully studied. It will be noticed that all the 

 superior and inferior boundaries are membranous, for here there is noth- 

 ing to prevent a considerable extension of growth, while in the inner- 

 most portions, where pressure comes from practically all sides, it is 

 cartilage. 



Babies have a very soft spot on top of and close to the center of 

 the head for about one and one-half years after birth. Places such as 

 these are called fontanelles. These fontanelles are found during the em- 

 bryonic period at all spots in the skull where several points of ossifica- 

 tion come together. Ossification begins at many points, each center of 

 ossification extending and growing toward each other. The fontanelle 

 is the unossified spot that constantly becomes smaller until ossification 

 is complete. 



Professor Eben J. Carey has recently shown, that contrary to the 

 usually accepted idea that bone grows simply because there is an inner 

 something which makes it assume definite forms, that it is the stress and 



