104 



DEVELOPMENT OF BLOOD-VESSELS. 



appearances by this usually most accurate observer, or, what is less proba- 

 ble, to the existence of another process less simple than the former, by 

 which also the development of blood-vessels may be effected. Of the 

 correctness of Kolliker's account, the writer can' speak with complete 

 certainty, from having, in some investigations with Mr. Paget, observed aw 

 almost exactly similar series of changes in the fine gelatinous tissue con- 

 veying the umbilical vessels of a sheep's embryo seven lines in length to the 

 uterine cotyledons. Perhaps no better tissue than this could be selected for 

 witnessing the mode of development of blood-vessels, for it is exceedingly 

 fine, transparent, and composed almost entirely of a homogeneous substance, 

 in which numerous scattered cytoblasts and cells, with developing blood- 

 vessels, are almost the only objects seen. In some portions the develop- 

 ment of vessels is complete, networks of various sized tubes filled with 

 blood-corpuscles alone appearing. But in most other portions, together 

 with completely formed and permeable vessels, the various steps in the 

 development of these from elongating and branching cells, are distinctly 

 seen. In such places are observed chains and networks of cell-like bodies, 

 mostly filled with granular matter, and occasionally presenting a clear 

 oval nucleus, and connected to each other by exceedingly slender filaments, 

 some of which appear tubular, and in many instances are connected 

 with blood-vessels of considerable size (see fig. 22). The cell-like bodies 



thus connected are of 

 various shapes, most of 

 them round or oval, many 

 very narrow and spindle- 

 shaped, some angular and 

 elongated from their an- 

 gles. The threads of con- 

 nection are attached to 

 the angles and points of 

 the elongated bodies, and 

 in the case of the round 

 and oval ones, are so 

 attached that these bodies 

 appear like varicose or 

 aneurismal enlargements. 



The various transitional states, from the fine solid threads of con- 

 nection, to permeable tubules containing one or more rows of blood- 

 corpuscles, are very manifest. As observed by Kolliker, the formation 

 of a tubular cavity in the filaments appears, though not invariably, to 

 commence at the points of their attachment to permeable blood-vessels 

 and to the cell-like bodies. Occasionally blood-corpuscles may be traced 



* This and the following figure, for the use of which the writer is indebted to the kind- 

 ness of Mr. Paget, represent several of the appearances described in the text. 



