8 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Notochord of Mammals.* — Prof. H. Leboucq has examined 

 embryos of man, ruminants (lambs and calves), rodents (rats, mice, 

 guinea-pigs, but especially rabbits), and the mole, afterwards compar- 

 ing these with a few observations on the chick, for the purpose of 

 studying the retrograde metamorphosis of the notochord among the 

 higher vertebrates. The results of his predecessors are very briefly 

 noted. He strongly recommends the method of decalcification by 

 picric or nitric acid, indicated by Busch,! to all who have to deal with 

 osseous pieces of large extent. 



The mammalian notochord exhibits at least three phases of retro- 

 gression. During the first it is still continuous, its fusiform inter- 

 vertebral enlargements alternating with much narrower vertebral 

 portions. The former consist of two kinds of nucleated cells, central 

 and peripheral. The central cells are loosely united, with intervening 

 lacunaB. These lacunge cause the appearance of the lumen noted by 

 His.J The peripheral cells are closely approximated, as in an 

 epithelium. Cells of one kind only make up the vertebral portions ; 

 they are like the central cells of the inter vertebras. The skeleto- 

 genous tissue nearest the chorda shows also a certain alternation of 

 structure, and may be said to constitute a series of sheaths for its 

 vertebral (but not for its intervertebral) portions. The chorda dis- 

 played the structure just mentioned in a human embryo (of the eighth 

 week) 2 • 5 cm., and in one embryo of the cow 3 cm. long. In a younger 

 embryo of the cow, from 1 to 2 cm., Dursy found the intervertebral 

 constituents of the chord alternating with fusiform vertebral enlarge- 

 ments, and directed attention to the curiously changed proportions 

 which the two kinds of segments afterwards present. 



In the second stage the chorda is broken up. It now consists of 

 distinct intervertebral and vertebral segments. These last are best 

 seen in sections across the bodies of tiie young vertebrae. Each 

 appears as a nucleated reticulum with very little protoplasm, and is 

 surrounded by a perichordal sheath finely striated longitudinally. 

 Outside this is a layer of cartilaginous cells ; next comes the ossifying 

 tissue. Towards either end of the newly forming bone, as we find by 

 making a number of transverse sections, the chord vanishes ; but the 

 radiating disposition of the cartilaginous cells remains to indicate the 

 fact of its disappearance. The more conspicuous intervertebral seg- 

 ments are now further vacuolated, shortened, and stretched in diameter. 

 This increase is mainly due to the encroachment and blending of the 

 perichordal tissue. A simultaneous formation of giant (multinucleated) 

 cells takes place. By this invasion and dissociation, already begun in 

 the preceding phase, the segments lose their fusiform figure, becoming 

 irregular in shape while they increase in volume. The hypothesis of 

 Lowe, admitted by KiJlliker, that the intervertebral segments with- 

 draw by a sort of attraction the rest of the chordal substance into 

 themselves — is inconsistent with the presence of vertebral segments 

 during this stage. 



* Arch, de Biol., i. (18S0) pp. 718-736 (1 pi.). 



t Arch. Mikr. Anat., xiv. p. 480. 



X The author's reference should be to p. 48, not to p. 68, of the work of His. 



