466 



Prof. W. K. Parker. 



[Mar. 22, 



shortened during their secular development. It seems to me to be 

 probable that the Amphibian stock from which birds arose — becoming 

 Reptiles in their ascent, but spurning that intermediate stage — were 

 long, eel-like forms, not dissimilar to Amphiuma and Menobranchus 

 among the existing Urodeles. I will therefore state what evidence 

 there is of evolutional abbreviation in the development of the species 

 in existing Birds. 



Working with Foster and Balfour's ' Elements of Embryology ' 

 beside me, I was struck with one part of their description, and with 

 my own preparations showing the phenomenon. At page 157 we read 

 as follows : — 



" The notochord [in the Chick] is on the sixth day at the maxi- 

 mum of its development, the changes which it henceforward under- 

 goes being of a retrograde character. 



"From the seventh day onward it is at various points encroache 1 

 upon by its investment. Constrictions are thus produced, which first 

 make their appearance in the intervertebral portions of the sacral 

 region. In the cervical region, according to Gregenbaur, the inter- 

 vertebral portions are not constricted till the ninth day, though as 

 early as the seventh day constrictions are visible in the vertebral 

 portions of the lower cervical vertebras. By the ninth and tenth 

 days, however, all the intervertebral portions have become distinctly 

 constricted, and at the same time in such vertebral portions there have 

 also appeared two constrictions, giving rise to a central and to two 

 terminal enlargements. In the space therefore corresponding to each 

 vertebra and its appropriate intervertebral portion, there are in all 

 four constrictions and their enlargements." 



I had long ago noticed, figured, and described a similar monili- 

 form condition in the cephalic portion of the notochord in the Chick,* 

 and this observation set me speculating upon the dying out of the 

 axial segmentation in the region of the skull. 



Now this peculiar secondary and temporary segmentation of the noto- 

 chord is not equal throughout the whole chain of rudimentary ver- 

 tebras ; I can only find two beads in the sacral region, and none in the 

 caudal. 



Nevertheless, taking these beadings as a true historical record of 

 development, and allowing for them in such a bird as the Common 

 Swan (Gygnus olor), we get, hypothetically, a very long ancestral 

 form. In that bird there are thirty presacral, twenty-one sacral, and 

 thirteen caudal vertebras that are developed as distinct vertebral 

 segments of the axis. Then, if we treble the presacrals and double 

 the sacrals, we add eighty-one to the actual sixty-four of the modern 

 bird, and thus obtain more than twelve dozen — 145 — vertebras with 

 which to accredit the ancestral form. 



* ' Phil. Trans ,' 1869, Plate 81, figs. 2 and 7, and Plate 82, fig. 3, p. 771. 



