254 TRANSACTIONS OF THE CANADIAN INSTITUTE. [YoL. II, 



haematoblast amoeboid in outlines. Its cytoplasma is as yet undiffer- 

 entiated and it does not possess a membrane although the peripheral 

 portion gives evidence of its formation in the presence of a series of 

 regularly arranged granule-like bodies affording a sharply outlined 

 border. 



In Figs. 19, 20 and 21 we see the haematoblasts of a later stage 

 with much fewer yolk spherules and with specialization of form and 

 structure allied to that in the mature red corpuscle. The outline 

 is oval or elliptical and the peripheral portion is usually limited 

 by a clear hyaline, somewhat thick membrane while the cytoplasma is 

 differentiated into coarse or fine trabeculae strewn along which are 

 granules, some of them brownish in color like those found occurring in 

 the mesoblastic and ectoblastic cells of this and later stages. Frequently 

 the cytoplasma in the immediate vicmity of the nucleus is denser, stains 

 somewhat more deeply than the remainder while it sends coarse pro- 

 longations in a radiating fashion outwards (Fig. 20). The corpuscles are 

 not as yet flattened, but about the twentieth day the majority of them 

 are elliptical in outline and flattened. When the larvae of this date are 

 fixed with Flemming's Fluid the discoplasma and nuclei of such 

 blood cells are homogeneous, indicating that the latter are fully formed^ 

 or mature blood cells. These corpuscles are no longer capable of division 

 and their nuclei give with alum-cochineal a reddish-brown stain and with 

 haematoxylin a brown stain, in each case like that given in the red cor- 

 puscles of the adult animal. There still persist haematoblasts in which 

 karyokinesis is very common and in which no specialization of form, 

 such as that described for the remaining blood cells, is observable. These 

 are the elements from which originate, not only the future blood cor- 

 puscles, but also the future haematoblasts. These elements form but a 

 small proportion of the whole number of corpuscles and as they 

 possess the power of division while the mature elements do not, the 

 origin of these must now be considered. 



In order to determine this, sections of larva; of the eighteenth and nine- 

 teenth days hardened in chromic acid and stained with hematoxylin and 

 eosin must be examined. If a section through the sinus venosus be 

 under observation it will be found that that cavity contains a large 

 number of blood corpuscles which, according to the staining effects of the 

 two dyes, can be divided into two classes: one, the more numerous in 

 which both nucleus and cytoplasma show a special afifinity for the eosin, 

 the former being often stained only with this dye; the other, comprising 

 corpuscles in the nuclei of which the haematoxylin alone has reacted. 

 Both classes of corpuscles are fairly represented in F'ig. 15, « and ^, the 



