FORMATIOX OF BLOOD-CORPrSCLES. 



41 



In the bird. — In the egg of the bird, the first appearance of blood-corpuscles, 

 as well as of blood-vessels, is seen in the blastoderma, or germinal membrane, a 

 structure formed by the extension of the cicatricula. in the early stages of incu- 

 bation. The commencing embryo, with its simple tubular heart, is seen in the 

 middle of this circular membrane, and blood-vessels, containing blood-corpuscles, 

 appear over a great part of its area. These first vessels, therefore, though con- 

 nected with the heart, and intended to convey nutriment to the embryo, are 

 formed in an exterior structure ; but, in a somewhat later stage, blood-vessels 

 and corpuscles are developed in various textures and organs within the body. 

 The formation of blood-corpuscles in the middle layer of the blastoderm has been 

 recently carefully investigated by Klein.* He describes the blood-vessels of the 

 embryo chick as originating in an endogenous manner in the interior of certain 

 of the cells of the middle layer of the blastoderm. It would appear, first, that 

 the nuclei of these cells become multiplied, and that then the protoplasm 

 around each takes on a reddish colour, and, a cavity becoming formed within the 

 mother-cell by the enlargement of a vacuole, the newly-formed, nucleated, red 

 blood-corpuscles become free -n-ithin the cavity thus produced (fig. IG). In 

 other instances the cavity be- 

 comes first formed within the Fig. IG 

 cell, which is considerably en- 

 larged, and in the protoplasmic 

 wall of which nuclei are em- 

 bedded. From this wall, blood- 

 coii^uscles, both red and white, 

 bud forth into the interior of 

 the vesicle. The mother-cells 

 send out processes which connect 

 them with one another, and into 

 these processes their cavities are 

 eventually extended : in this way 

 a system of blood-vessels contain- 

 ing blood is produced. According 

 to Balfour f it is the nuclei them- 

 selves which become the coloured 

 corpuscles, whilst the nucleoli 

 within them develop into the so- 

 called "nuclei" of the blood- 

 corpuscles. 



It is uncertain whether any of 

 the i^rimary red corpuscles are 

 formed bj' direct transfoiTnation 

 of embr\-onic cells, as described 



in the eml^rj-o of Batrachians. At the same time they agree with those cells in 

 exliibiting amoeboid movements. 



In man and mammalia. — In the embryo of man and mammalia the 

 primitive red blood-corpuscles are nucleated spheroidal bodies, of much larger 

 size than the future red disks. As to their origin nothing is certainly known : 

 they are prolmbly transformed embryonic cells. These large nucleated red and 

 colourless coiijuscles. continuing to increase in number, constitute the earUest, 

 and, for a time, the only corpuscles in the embryo-vessels. But their multiplica- 

 tion is soon arrested, and a new epoch in blood-fomiation begins with the 

 development of the liver. The blood which returns to the embryo charged with 

 fresh material of nutrition from the maternal system, has then to pass, at fii'st 

 entirely aftem-ards in great part, through the vessels of the liver ; and it would 

 seem that henceforth colourless nucleated corpuscles are produced in that organ 

 and poured abundantly into the general mass of blood by the hei^atic veins. It 

 is probable that the liver continues its ha^mapoietic or blood-forming function 

 throughout fcetal life ; but, in the meanwhile, the sjDleen and lymphatic system 



Fig. 16. — YAniofs Forms of Mother-Cells 



UNDERGOING DEVELOPMENT INTO BloOD-YESSELS 



(from the middle layer of the chick's blastoderm. 

 Klein.) 



d, d, blood-corpuscles. 



* Wiener Sitzimgsberichte, Ixiii. 1871. 

 t Quarterly Journal of ^Microscopic Science. 



July, 1873. 



