ii8 ANIMAL BIOLOGY. [Parti. 



Blood. In the vascular area of the blastoderm of fowl and 

 rabbit some of the nucleated mesoblast cells become united one 

 with another by protoplasmic processes, so that an irregular 

 network of nucleated cells is produced. The nuclei of the cells 

 rapidly multiply by division. Some remain central, and the 

 protoplasm which surrounds each of these acquires a yellowish- 

 red colour. Vacuoles are formed in the protoplasmic network 

 in which they lie, and the vacuoles running together, there 

 results a system of branched canals enclosing a fluid in which 

 the nucleated corpuscles float in irregular clusters. Eventually 

 these clusters break up into a number of red corpuscles, each 

 with an internal nucleus and surrounding red-tinted protoplasm. 

 These primitive red corpuscles seem to be capable of amoeboid 

 movements like the white corpuscles, and of multiplication by 

 division. Thus is formed a network of canals the primitive 

 blood-vessels of the blastoderm containing red blood-corpuscles. 

 It is probable that within the embryo itself primitive blood- 

 corpuscles and blood-vessels are developed in a similar way. 

 The protoplasmic walls of these primitive blood-vessels at 

 first contain only a few nuclei irregularly imbedded in the 

 protoplasm. But subsequently the protoplasm around these 

 nuclei and their descendants becomes differentiated into the 

 flattened cells which compose the walls of the blood-capil- 

 laries, and which form the lining membrane of the larger vessels 

 in the adult. Around this epithelioid layer in the larger 

 vessels, connective tissue and muscular coats are subsequently 

 formed. 



The primitive nucleated red blood-corpuscles of the mammal 

 are subsequently converted into the non-nucleated blood-discs, 

 losing their nucleus and shrinking in size. Thus the nucleated 

 red corpuscles are, during embryonic life, gradually replaced by 

 non-nucleated red discs. But there can be no doubt that red 

 discs are also produced in other ways. They are formed, for 

 example, by the separation of masses of protoplasm from the large 

 cells of the red marrow of bones. It has also been shown that 

 white corpuscles develop into nucleated red corpuscles, and 

 these into red discs. The white corpuscles would seem to be 



