'^.")v (u'lu'tic I iitci'prctnt ions in llic |)(iiii;iiii of Anatoiny 



tidiis. tlinsc iiircct iiii:' the iii;iti-i.\, aiid lliosc nll'cci iiiii' the cells. On these 

 (liircrcuccs tlic clnssilical i<ni in the Inllowiiiu- tahic is hascd. 



We also know lliat eoimeetixe tissue can l)e diret'tly 1 ransfornicd into 

 cartilage, which. Ihererore. iiii(|uestional)]y helongs in the same second 

 genetic gT()ii|) as the true connective tissues. .\s regai'ds hone, 1 find it 

 souiewliat dilHciilt to i-eaeh a decision, hut incline to the conclusion that 

 hone should he regarded as distinct from the true connective tissue, thus 

 nuiking a third genetic division of the tissues derived from the primitive 

 mesenchynia. 'I'liis conclusion a])i)eals to me partly as a protest against 

 the ahsurd, though long estahlished and honored, custom of separating 

 cartilage from connective tissue, and putting cartilage and hone together 

 in a common grouj) under the head of supporting tissues. The following 

 tal)le presents the ])i'o])osed classification in a form which you can easily 

 follow : 



MESENCHYMA.i 



TABLE I.— THE MESENCHYMAL TISSUES. 



Cellular reticulum TMucous tissue 



' Matrix specialized.-^ Adult connective tissue 

 I Cartilage 



Embryonic connective 

 tissue ■ 



Cells specialized 



.Bone 



Fat cells 

 Pigment cells 

 Smooth muscles 

 Basement membranes 

 Pseudo-endothelium 



I Genital interstitial cells, 



L etc. 



Let me refer hriefiy to a third and more special example of the 

 genetic t-lassification of tissues, namely, that of the hlood-vessels. As you 

 probably all know% recent embryological investigations have compelled us 

 to recognize not only the three familiar and long-known classes of blood- 

 vessels, arteries, veins, and capillaries, but also a fourth class, that of the 

 sinusoids. Capillaries arise as small vascular sprouts from pre-existing 

 vessels, and these sprouts grow in the mesenchyma. A sinusoid, on the 

 contrary, has an entirely dift'erent developmental history, for it is ]u-o- 

 duced by the subdivision of a ])re-existing and relatively large vessel. 

 The sul)division is accom])lished by the ]u-oliferating tubules (or trabec- 

 uUp) of an organ, which encounter a large vessel and invade its lumen, 

 pushing the endothelium before them.'' The endothelium of the vessel, 

 on the other hand. ex])ands and spreads over the tubules (or tral)ec-uhv)- 

 By the convolutions and anastomoses thus produced, a large vessel is sub- 

 divided into small ones. It follows that a sinusoi(hil circulation is 

 ])urely venous or ])urely arterial, it may sultit'c. upon this occasion, to 

 ]toint out again that the structure of numy impoi'tant organs, as for 



