DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 



HISTORY OF THE PARABLAST- AND MESENCHYME-THEORIES. 



The older investigators, as, for example, REMAK, grouped together all the 

 cells which are inserted between the two primary germ-layers under the 

 common name of the middle germ-layer, and assumed for them a common 

 origin. To this conception His opposed in the year 1868 in " Die erste Ent- 

 wicklung des Hiihnchens im Ei " his " parablast -theory" in which, influenced 

 principally by histogenetic considerations, he distinguished two fundaments 

 of different origin, an archiblastic and a parablastic. 



As archiblastic fundament he designated the part of the middle germ-layer 

 which lies in the body of the embryo itself, the axial cord (Achsenstrang) and 

 the animal and vegetative muscle-plates, and he made them arise by de- 

 lamination from the primary germ-layers, and therefore ultimately from the 

 embryonic cleavage-cells. 



He gave the name parablastto a peripheral fundament, lying originally outside 

 the embryo, which is the source of all the connective substances, the blood and 

 the vascular endothelium, and which grows from the margin, or more speci- 

 fically from the opaque area, into the body between the archiblastic tissues. 



The division of the middle germ-layer into archiblast (chief germ) and 

 parablast (accessory germ), proposed by His and carried out in several of his 

 writings, found at the time no approbation, and encountered decided and 

 successful opposition, especially on the part of HAECKEL. because the correct 

 views contained in the doctrine were obscured and covered up by peculiar 

 conceptions about the origin of the parablast. The parablast, it was claimed, 

 is not derived from the egg-cell, but from the white yolk, a product of the 

 granulosa-cells, which, according to the earlier teachings of His, penetrate 

 into the primordial ovum in great numbers and become the white yolk-cells 

 and the yellow spherules. But the granulosa-cells in turn, it was maintained, 

 arise from the connective tissue (leucocytes) of the mother ; consequently 

 after their migration into the egg they are capable of producing again 

 only connective tissue and blood. 



His thought it was necessary to assume a fundamental difference between 

 chief germ and accessory germ ; the former alone had experienced the influence 

 of fertilisation, since it alone was descended from cleavage-cells, whereas the 

 latter, since it issued from the white yolk (a derivative of the maternal con- 

 nective tissue), was " purely a maternal dower." 



RAUBER, in a short communication, accepted the conclusions of His, in so 

 far as he also assumed a common origin for blood and connective tissue, a 

 special " baemo-desmoblast," but differed from him in that he derived them 

 from the cleavage-cells. 



GOETTE (1874) is also to be mentioned in this connection, since he maintained 

 that the blood is developed out of yolk-cells, which break up into clusters of 

 smaller cells (Amphibia and Birds). 



Proceeding from other standpoints, and induced by observations on In- 

 vertebrates, my brother and 1 were led in our Ccelom- Theory (1881) to a result 

 similar to that of His, namely, that two entirely different structures had been 

 hitherto embiaced under the expression middle germ-layer, and that it was- 

 necepsary to introduce in the place of the old indefinite conception two new 

 and more precise ones, " middle germ-layer in the restricted sense " and " mesen* 

 chyme-germ'' But our conception, notwithstanding many points of agree- 

 ment, took in detail a form very different from the doctrine of His. 



