(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." _Zool. Anzeiger_, Vol. II. 1879, pp. 520, 593, 612. (247) R. Remak. _Untersuchungen üb. d. Entwicklung d. Wirbelthiere_, 1850-1858. (248) A. Rauber. _Primitivstreifen u. Neurula d. Wirbelthiere._ Leipzig, 1877. CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA. The present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by comparative anatomy, the characters of the ancestors of the Chordata; and to trace as far as possible from what invertebrate stock this ancestor was derived. The second of these questions has been recently dealt with in a very suggestive manner by both Dohrn (No. 250) and Semper (Nos. 255 and 256), but it is still so obscure that I shall refrain from any detailed discussion of it. While differing very widely in many points both Dohrn and Semper have arrived at the view, already tentatively put forward by earlier anatomists, that the nearest allies of the Chordata are to be sought for amongst the Chætopoda, and that the dorsal surface of the Chordata with the spinal cord corresponds morphologically with the ventral surface of the Chætopods with the ventral ganglion chain. In discussing this subject some time ago[100] I suggested that we must look for the ancestors of the Chordata, not in allies of the present Chætopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chætopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to its existence is well founded, appears now to have perished. The recent researches of Hubrecht on the anatomy of the Nemertines[101] have, however, added somewhat to the probability of my views, in that they shew that in some existing Nemertines the nerve-cords approach each other very closely in the dorsal line. [100] _Monograph on the development of Elasmobranch Fishes_, pp. 170-173. [101] Hubrecht, "Zur Anat. u. Phys. d. Nervensystems der Nemertinen." _Kön. Akad. Wiss. Amsterdam_; and "Researches on the Nervous System of Nemertines." _Quart. Journ. of Micr. Science_, 1880. With reference to the characters of the ancestor of the Chordata the following pages contain a few tentative suggestions rather than an attempt to deal with the whole subject; while the origin of certain of the organs is dealt with in a more special manner in the chapters on organogeny which form the second part of this work.