88 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



the form of the widely distributed mesostroma. The obstacles which this last- 

 named tissue offers to the countless tiny currents resulting from cell activity must 

 be almost nil. It is in large measure in these Lilliputian forces, as well as in the 

 various and varying chemical changes which are constantly going on, that one 

 must seek the explanation for the earliest vasculogenic conditions. Cell growth 

 and migrations, cell form and orientation, vacuolation and the formations of cysts 

 and definite channels are all, in part, the local reaction to a present stimulus. Endo- 

 thelium comes into existence in response to certain influences, and in the face of new 

 conditions it may again revert or give rise to the indifferent tissue from which it 

 sprang. Even after the establishment of a complete, closed circulation, the wide 

 intercellular, often avascular spaces of the embryo must play an important role in 

 the exchange of various fluids. Gradually, however, this feature becomes less con- 

 spicuous, but in the last analysis the adult conditions are exactly the same, where 

 the individual cells live, move, and have their being in the same primitive way, in 

 the tissue fluids which bathe them. 



From the very first there has been what we may call a physiological circula- 

 tion, howbeit very primitive, but nevertheless quite sufficient. The necessary 

 interchanges and transference of various substances are soon facilitated and accel- 

 erated by the gradual appearance of freer pathways and more sharply limited 

 channels in definite localities. These early vessels, with their thin or at times 

 deficient walls, must confer a material benefit upon the adjacent regions, even 

 before the heart is capable of strengthening or directing the feeble currents within 

 them. Soon, of course, the early, primitive arrangements must give way before 

 the powerful pulsations of a functionating heart, which remains henceforth a sine 

 qua non of further development. 



That the vascular system, as we find it here, can not be injected goes without 

 saying. In the first place there is a most conspicuous lack of continuity, and even 

 where present the vessels do not always present a definite wall separating their 

 lumina from the wide intercellular spaces without. Extravasations under any 

 conditions of intravascular pressure would obviously be a foregone conclusion. 

 One speaks of extravasation because the starting-point is the lumen of the vessel; 

 but here, in many cases, the vessels might be injected from the loose mesostroma 

 around them. The differences between vessel walls and deficiencies in the same 

 are morphological rather than physiological, distinctions of degree rather than of 

 quality, for in both instances there is an interchange of material in both directions. 



We would take occasion at this time to express our thanks to Dr. George L. 

 Streeter for so generously placing the facilities of the Carnegie Laboratory at our 

 disposal. Appreciation is also due to Mr. 0. 0. Heard for invaluable aid rendered 

 in preparing the models from which the accompanying illustrations were made; 

 and to Mr. James F. Didusch for the excellent maimer in which these illustrations 

 were carried out. 



