386 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
embryo with what may be seen in several other important Vertebrate types ; in none 
is this comparison more instructive than in that of the Selachians with the Snakes. 
(See Trans. Zool. Soc., Vol. 10, plates 34-42, pp. 189-234 ) Moreover, Mr. F. M. 
Balfour’s researches on the embryology of the Sharks and Skates 4 ' begin to shed a 
most welcome light upon the development of those most instructive fishes from the 
blastoderm ; his work dovetails with mine in the most convenient manner. But for the 
development of the “ Sauropsidan ” embryo the reader is referred to a joint work by 
Professor Foster and Mr. F. M. Balfour — their £ Elements of Embryology,’ Part I., 
1874. So completely at one are the processes by which a Snake and a Bird — the root 
and the branch of the great group of the Sauropsida — are developed, that these 
accomplished authors might have taken the Snake and not the Bird as their subject ; 
scarcely a single figure would need to be altered, if a counterpart work, taking the 
Snake as the subject, were to be prepared and published. My earliest snake-embryo 
(Plate 27, figs. 1, 2) corresponds very accurately with that of the Fowl at the end 
of the fourth day of incubation (Foster and Balfour, p. 142, fig. 46) ; the allantois 
in my specimens had exactly the relative size shown in them figure of the chick 
at that date. 
I shall, once for all, refer the reader to Rathke’s descriptions of the early stages of 
the skull (both in the original and in Professor PIuxley’s translation), and use my 
own terms and modes of speech in endeavouring to make the matter plain. 
First Stage. Embryo Snakes, § of an inch in absolute length. 
Notwithstanding the length of these embryos, the testa of a mustard seed would 
have sufficed to enclose one of them ; the figure (Plate 27, fig. 1) shows only the root 
of the tail, which is tucked in amongst the folds of the body ; the neck is partly 
unwound, to display the huge bead better. 
The embryonic membranes are now perfect, and the embryo, on the whole, is a 
finished form ; yet the huge loop-shaped heart is only covered by the pericardium, 
as it lies in the re-entering angle of the throat (fig. 1, h., pcd.). 
The head is bent upon itself at an angle which is less than a right angle (Plate 27, 
fig. 1; Plate 29, fig. 1); this great degree of flexion is retained even in the next 
stage (Plate 27, fig. 3, and Plate 29, fig. 2). 
Of the three brain-vesicles, the hindmost (C 3 ) is the longest, and is least elevated; 
the fore brain (C 1 ) is becoming lobulate by reason of the budding out of the rudiments 
of the hemispheres ; and the mid brain (C 2 ) is most regular in shape. 
There are large spaces in each division, filled with a glairy fluid (Plate 29, fig. 1), 
and the more solid tract is of great length in the hind brain. 
The hollow space within the loop of the brain (“ middle trabecula ” of Rathke) is 
* Journ. of Anat. and Pliys. (a series of most important papers), and PM1. Trans., 1876, Part I., 
Plates 16—18, pp. 175-195; and his work on the ‘ Elasmobranchs,’ 8vo., Macmillan and Co., 1878. 
