250 Boyal Society: — 



already appeared on other Vertebrate Skulls. I was led to work 

 out this MEDIUM TYPE, and not a more generalized form, such as the 

 Guinea-pig (see " On the Development of the Frog's Skull," Phil. 

 Trans. 1871, p. 203), through the circumstance of an offer from my 

 friend Mr. Charles Stewart to put some seventy embryos of the 

 Common Pig into my possession. In the present communication I 

 have had the invaluable help of adA^iee and oversight from Pro- 

 fessor Huxley; whilst the labour of my hands has been lightened 

 by my son, Mr. T. J. Parker, who prepared for me all the more 

 delicate sections. The embryos ranged in size from two thirds, 

 or less, of an inch in length, with the head only equal in size to 

 a siveet p&a, whilst the head of the largest specimen was the size 

 of that of the Common Squirrel. To these I have added young 

 pigs at birth, and have taken as the last stage the skull of a half- 

 grown indi\idual. 



The most important results of the present investigation may be 

 stated as follows : — 



1. In a pig-embryo, in which the length of the body did not 

 exceed two thirds of an inch, and four postoral clefts were present, 

 the cranio-facial skeleton was found to consist of : — (a) the noto- 

 chord, terminating by a rounded end immediately behind the pitui- 

 tary body. 



(6) On each side of the notochord, but below it, there is a cartila- 

 ginous plate, which in front ends by a rounded extremity on a 

 level \^dth the apex of the notochord, while behind it widens out 

 and ends at the free lower margin of the occipital foramen. These 

 two plates, taken together, constitute the " investing mass " of 

 Eathke. In this stage they send up no prolongations around the 

 occipital foramen ; in other words, the rudiment of the basioc- 

 cipital exists, but not of the exoccipital or superoccipital. 



(c) The large oval auditory capsules lie on each side of the 

 anterior half of the investing mass, with which they are but imper- 

 fectly united : there is no indication of the stapes at this stage. 



id) The trabecular or first pair of preeoral visceral arches in- 

 close a lyre-shaped pituitary space ; they are closely applied to- 

 gether in front of this space, and, coalescing, give rise to an 

 azygous prsenasal rostrum. They are distinct from one another 

 and the investing mass. 



{e) The ptery go-palatine or second pair of visceral arches lie 

 in the maxillo- palatine processes, and are therefore subocular in 

 position. Each is a sigmoid bar of nascent cartilage, the incurved 

 anterior end of which lies behind the internal nasal aperture, while 

 the posterior extremity is curved outwards about the level of the 

 angle of the mouth. The pterygo-palatine cartilages are perfectly free 

 and distinct from the first prseoral and from the first postoral arch. 



(/) The mamlihular or first pair of postoral \asceral arches are 

 stout continuous rods of cartilage which lie in the first ^dsceral 

 arch behind the mouth. The ventral or distal ends of these arches 

 are not yet in contact ; the dorsal or proximal end of each is 

 somewhat pointed and sharply incurved, pushing inwards the 



