ORIGIN OF NUCLEATED RED BLOOD-CORPDSCLES OF THE EMBRYO. 217 



Pale blood-corpuscles also, which have migrated from the vessels, may find their way 

 into the beginning of the lymphatics. In this way the presence of corpuscles in the 

 lymph even before it has passed through the lymphatic glands is accounted for. 

 Lymph-corpuscles are also produced in the spleen and in the thymus gland (the 

 latter in early life) ; and it is still believed by some authors that they may also be 

 formed by proliferation of connective tissue corpuscles. The corpuscles of the chyle 

 and lymph are carried into the sanguiferous system and become the pale corpuscles 

 of the blood, but some of the latter may pass directly from the lymphatic glands, 

 spleen, and other organs containing lymphatic or lymphoid tissue into the blood- 

 vessels which are supplied to those organs. 



Origin of the nucleated red blood-corpuscles of the embryo. The first 

 red blood-corpuscles are formed very early in embryonic life simultaneously with and 

 in the interior of the first blood-vessels. They are developed in the mesoblast, in a 

 circular area which surrounds the part of the blastoderm which is occupied by the 

 developing body of the embryo. The area is known as the vascular area, and the 

 first blood-vessels and blood-corpuscles are, therefore, formed outside the actual body 

 of the embryo. The process of development is as follows : 



Those mesoblastic cells in the vascular area which are concerned with the for- 

 mation of vessels (angioblasts) become extended into processes of varying length, 

 which grow out from the cells in two or more directions. The cells become united 

 with one another, either directly or by the junction of their processes, so that an 



Fig. 254.. PART OF THE NETWORK IN DEVELOPING BLOOD-VESSELS IN THE VASCULAR AREA OF THE 



GUINEA-PIG. (E. A. S.) 



U, blood-corpuscles becoming free in an enlarged and hollowed out part of the network. The 

 smaller figure on the left represents a of the larger figure, more highly magnified ; d, a nucleus under- 

 going division. 



irregular network of protoplasmic nucleated corpuscles is thus formed (fig. 254). Mean- 

 while the nuclei become multiplied, and whilst the greater number remain grouped 

 together in the original cell-bodies or nodes of the network, some are seen in the 

 uniting cords. The nuclei which remain in the nodes accumulate, each one around 

 itself, a small amount of cell-protoplasm. The corpuscles thus formed (bl) acquire 

 a reddish colour, and the protoplasmic network in which they lie becomes 

 vacuolated and hollowed out into a system of branched canals enclosing fluid, in 

 which the nucleated coloured corpuscles float. The intercommunicating canals 

 gradually become enlarged so as to admit of the passage of the corpuscles. The 

 protoplasm which forms the wall of these first vessels becomes differentiated around 

 the nuclei which have remained embedded in it, so as to give rise to the flat cells 

 which compose the blood-capillaries. 



As soon as the heart is developed, or even before this happens, the blood begins 



