DEVELOPMENT OF THE HORSE. 15 



advanced to admit of milk being pumped from the mammary 

 glands into its gullet or oesophagus. The figure indicates the 

 exact size (just under 1]- inches) of the embryo. The open 

 mouth and well-formed lips and tongue seem to indicate that it 

 is quite ready (should a huge reversion or throw-back take 

 place) to receive and hold on to a teat. There is, in fact, in the 

 seven-weeks' embryo not a little in favour of the view that all 

 animals climb their own ancestral tree, — that there is a more or 

 less accurate recapitulation by each individual of the entire 

 ancestral history. 1 I have no doubt the remote ancestors of the 

 horse were born ere this stage was reached. If the rate of pro- 

 gress was the same in the remote past as it is now, the birth 

 would originally occur on the forty-seventh or forty-eighth day. 

 As the figure shows, the head is less flexed than at six weeks, and 

 the nose, eyes, and ears are better formed, and that it is beyond 

 doubt a mammalian embryo is proved by the presence of rudi- 

 ments of hair on the snout and eyelids. But any embryologist 

 would not only at once place this embryo in the mammal group, 

 he would have no hesitation in asserting it belonged to the horse 

 family. The fore and hind limbs clearly point to this, for already 

 the frog is partly modelled, and the fetlock is quite distinct ; and 

 while the position of the knee and elbow are evident in the fore 

 limb, the position of the hock and stifle are equally evident in 

 the hind ; and, further, the possibility of having more than one 

 complete useful digit for each foot no longer exists. Moreover, 

 even without the aid of a lens, two teats can be detected, and 

 hence it would be possible, without knowing anything of the 

 history of this embryo, to state that it had been obtained from 

 some member of the horse family, and that it would probably 

 have developed into a filly foal. 



On turning to the appendages, it will be observed that though 

 the yolk sac has lost little in capacity, its connection with the 

 embryonic sac has been considerably reduced. The absorbing area 

 is now very small (a-c, fig. 6), so small that it is difficult to 

 understand how sufficient nourishment has been obtained to 



1 That there is not an exact, or anything approaching an exact, recapitulation, 

 a glance at the seven-weeks' embryo will at once show. The remote ancestors 

 had five complete digits, but even in the seven-weeks' horse there is no external 

 indication of more than a single disdt for each foot. 



