GENERAL INTRODUCTION. XXXIX 



very backward state in the young horse at the end of 

 the sixth week, it is still greatly shorter than the upper 

 jaw at the beginning of the forty-seventh day. Evidently 

 during the last two days of the seventh week the develop- 

 ment of the structures around the mouth proceeds apace, 

 as if it were unexpectedly necessary to prepare the em- 

 bryo for premature birth, and for playing the part of a 

 helpless opossum. By the end of the forty-ninth day 

 the lower jaw decidedly projects beyond the upper, 

 the tongue is well formed and slightly raised from the 

 floor of the mouth, which is sufficiently open to show a 

 fair-sized, well-formed space between its roof and the 

 well-moulded tongue. A point of still greater interest 

 is that both the upper and lower lips are so notched that, 

 if approximated, a gap would be left about the size of 

 the slender terminal portion of the teat in a Marsupial. 

 Another point is that, as the lips rapidly develop, rudiments 

 of the vibrissas (sensitive hairs) make their appearance 

 around the muzzle. As I said before, all these changes 

 taking place about the time the horse would presumably 

 be born were it a Mai'supial may be a mere coincidence, 

 and quite meaningless. They may, however, point to a time 

 when the ancestors of the horse were as primitive as some 

 of the Australian Marsupials are to-day, to a time when 

 they were prematurely ushered into the world in a per- 

 fectly helpless condition ; and they may further indicate 

 that, if the higher Mammals (the Eutheria) have not 

 actually sprung from amongst the Marsupials, they and 

 the ancient Marsupials travelled for a time along nearly 

 parallel lines. If this is the explanation, it follows that 

 at the end of the seventh week (when the horse embryo 

 is little over an inch in length) we have an extremely 

 interesting example of recapitulation, a kind of embryonic 

 reversion. Evidently it is but a passing phase, for ere 

 the eighth week comes to an end the mouth is tightly 

 closed — the lower lip partly overlapping the upper at each 

 side — and the eyes are also closed. The eyes, I might 

 have mentioned, are widely open during the whole of the 

 seventh week. 



