36 The MicRoscoPE. 



This now brings us to the consideraticn of the various ways 

 in which bone is developed. Different authors classify the de- 

 velopment of bone differently, but as it is the intention of this 

 paper to show the difference as a whole between bone and another 

 substance, the selection of two classes of development will suffice. 

 The selection which has been made and illustrated in Figs 1 and 

 2, represents a type of intramembranous and intercartilaginous 

 development. In both figures the calcospherules and contained 

 osteoblasts are plainly seen. In Fig. 2 the contour of the cells 

 is more regular and at the left of the figure cartilage cells may 

 be noted. 



After briefly noticing the structure and development of bone, 

 let us ask the question. Whence originates the osteoblast? Sud- 

 duth says, "In the first instance I am pretty fully convinced 

 that they are the ordinary embryonic connective tissue cells." 

 Here he refers to his classification of " Interstitial development ;" 

 again, " In some instances osteoblasts are produced from con- 

 nective tissue cells ; sometimes they have their origin in cartil- 

 age cells; " and again, " Connective tissues arise from the meso- 

 blastic layer of the blastoderm." One thing seems to be con- 

 clusive, that all forms of bone development are the product of 

 the mesoblast or middle layer of the blastoderm. In the study 

 of the enamel organ we shall first notice that it originates from 

 the epiblastic or outer layer of the blastoderm. The two laj^ers 

 which originally comprise the blastoderm are known as the 

 epiblast and hypoblast. Very soon after incubation in an egg 

 another layer known as the mesoblast appears ; from this layer 

 as we have already noticed, bone is derived. It remains for us 

 now to consider one of the interesting products of the epiblast. 



The nails, hair, sebaceous glands, enamel organ, as well as the 

 epidermis, are alike products of the epiblast layer. No one has 

 yet been able to say why the cells of this same layer should 

 produce hair in one place, nail in another, sebaceous glands in 

 another and the mucous membrane and teeth with the dense 

 enamel in very close proximity elsewhere. Especially when in 

 its incipiency the epiblast bears such a striking resemblance to 

 the adjacent mesoblast, yet if the process of development be not 

 interfered with, the greatest contrast results in their product- 

 At an early period differences can be shown between these layers. 

 The epiblast stains more readily and darkly than the mesoblast, 



