growtii in length, but they are eventually ossified 

 and united with the shaft as the marrow cavity 

 of the latter extends out into them. . . . 



". . . The marrow cavity is enlarged by the 

 chondrolytic action of all the vascular elements, 

 but chiefly by the walls of the budding blood ves- 

 sels. As the chondrocytes are liberated from 

 their enclosing matrix, they rapidly degenerate 

 and are apparently not converted into marrow 

 cells as previously believed; tlie cells of the mar- 

 row are presumably brought in from the mesen- 

 chyme and periosteum by the intrusive blood 

 vessels. 



"In birds, calcification does not precede ab- 

 sorption of the cartilage, as it does in mammals, 

 luitil the greater part of the marrow cavity is 

 formed. The cones of cartilage, referred to 

 above, that ar^e continuous with the articular 

 cartilages, are absorbed about ten days after 

 hatching." 



The histologic details of cartilage and bone 

 formation in the appendages of the embryo chick 

 have been described by Dantschakoff (1909b). 

 The marrow cavity appears in the hvmierus, tibia 

 and femur on the eighth to ninth day of incuba- 

 tion. She finds that the initial perforation into 

 the region of the marrow cavity comes from the 

 action of osteoclasts breaking through the thin 

 wall of newly formed bone. The opening per- 

 mits the entrance of surrounding mesenchyme 

 cells. 



She divides marrow cavity production into two 

 phases (p. 876) : 



"In der Entwicklung des Knochenmarks bei 

 den Vogeln gibt es eine gewisse, ziemlich lange, 

 vom 9, bis 12. Tage dauernde Periode, wah- 

 rend welcher seine Struktur und seine Differen- 

 zierungsprozesse sich so sehr von dem endgiil- 

 tigen Zustand am Ende des fetalen Lebens unter- 

 scheiden, dass es wohl berechtigt ist, wenn ich 

 diese Periode von den iibrigen trenne, und das 

 Mark wahrend derselben als primitives Knochen- 

 mark bezeichne. Das hauptsachlichste Unter- 

 scheidungsmerkmal dieser Periode ist das voU- 

 standige Fehlen einer Blutbildung in den Gefas- 

 sen, diese letzteren dienen zu dieser Zeit bloss zur 

 Ernahrung des Gewebes und enthalten zirkuli- 

 erendes Blut, welches aus mehr oder weniger 

 reifen Elementen besteht. . . ." ^ 



The description of these first vessels is as fol- 

 lows (p. 877): 



"Die Blutgefasse haben im primaren Knochen- 



mark eine sehr charakteristische Lage. In der 

 Mitte, parallel der Langsachse der Markhohle, 

 verlauft eine Arterie, die sich in den aussersten 

 Teilen der Hohle stark enveitert und dann in 

 zwei noch breitere diinnwandige Venen iibergeht. 

 Ein Unterschied im Inhalt der beiden Gefasse 

 existiert vorlaufig nicht. Die Kapillaren sind 

 sparlich." " 



The histogenesis of bone has been a secondaiy 

 consideration in this study, but osteoblasts and 

 osteoclasts were encountered during the search 

 for early stages of blood-cell development. 



When the femur of a chicken, incubated about 

 12 days, is split open so that a plug of marrow 

 projects beyond the spicules of bone and this is 

 touched to a slide, cells of the types shown in fig- 

 ures 319 and 320 are found. The embryo that 

 produced the cells shown in figure 319 was a few 

 hours younger than the embryo that produced 

 figure 320. In all bone-marrow smears made at 

 this age an abundance of serum granules as in 

 figure 322, is always present. Numerous smears 

 were made to get one sufficiently clear for de- 

 tailed study. For this reason, the low-power 

 drawings in figures 319 and 320 are composites. 

 Serum granules were present in the fields from 

 which these two plates were made but were 

 omitted in the drawing. 



The least differentiated cell in bone-marrow 

 smears from embryos of 11 to 12 days of in- 

 cubation is the type shown in cell 1 of figure 319. 

 Figure 320 shows three additional cells belong- 

 ing to this category. Cells 1 and 3 of figure 320 

 are typical of the early osteogenic cell and cell 

 2 is somewhat more differentiated. The lightly 

 stained cytoplasm is the most characteristic fea- 

 ture, along with a pattern of scattered, basi- 

 chromatin granules that are delicately stained. 



' Translation : In the development of tlie bone marrow in 

 birds there is a certain rather long period, lasting from the 

 9th to the 12th day, during which its structure and its process 

 of diflerentiation are so distinct from the final circumstances 

 at the end of the fetal life that there is good reason for me to 

 separate this period from the rest, and to designate the mar- 

 row during this time as primitive bone marrow. The most 

 important mark of distinction of this period is the complete 

 lack of a hematopoietic process in the vessels; at this time these 

 serve only to nourish the tissue and contain circulating blood, 

 which consists of more or less mature elements. . . . 



'Translation: The blood vessels have a very characteristic 

 location in the primary bone marrow. In the middle, parallel 

 to the longitudinal axis of the marrow cavity, there runs an 

 artery that widens greatly in the outermost portion of the hol- 

 low and then merges into two still wider thin-walled veins. 

 At the time there is not any distinction in the contents of the 

 two vessels. The capillaries are scanty. 



151 



