402 



Mr. W. K. Parker on the Structure [June 19, 



The first of these is the fact that, generally speaking, spots in the north 

 hemisphere have much about the same latitude as those occurriug at the 

 same, or nearly the same, period in the south, both sets widening or con- 



K tracting together. We may perhaps, therefore, suppose, by applying this 

 law, that the latitude of the spots which cause the positive extremes in 

 the above series is not greatly different from that of those which cause 

 the corresponding negative extremes. 



I s - The .second observational law is that which tells us that spots about the 

 same period have a tendency to attain their maximum at, or near, the same 

 , ^ ecliptical longitude. Now, if we suppose that in the foregoing three 

 series the greatest positive extremes were caused by the positive spots 

 attaining their greatest size, and the greatest negative extremes by the 

 negative spots attaining their greatest sizeyit would follow that the two 

 sets, positive and negative, must have taken their rise at places on the 

 sun's surface 180° of longitude different from each other, inasmuch as 

 the one set about 12 or 13 days before, or after, passed (let us say) the 

 same ecliptical longitude as the other. 



But if the positive set have the same latitude as the negative, and if 

 the one is 180° of solar longitude different from the other, it would 

 mean that the two outbreaks are at opposite ends of the same solar diameter. 



^ This conclusion is an interesting one ; but, of course, it requires to be 

 verified by further observation before it be finally received. Meanwhile 

 we are engaged in mapping out systematically the positions of the 

 various outbreaks upon the sun's surface, and we shall soon, therefore, 

 be able to find whether or not there be any truth in this conjecture. 



XIII. " On the Structure and Development of the. Skull in the 

 Pig (Sus scrofa)." By W. K. Parker, F.R.S. Received 

 May 17, 1873. 



(Abstract.) 



I have for some years past determined to concentrate my attention on 

 some one type of Mammalian Skull, so as to be able to present to the 

 Royal Society a paper similar to those which have already appeared on 

 other Vertebrate Skulls. I was led to work out this medium type, and not 

 a more generalized form, such as the GTuineapig (see " On the De^lopment 

 of the Prog'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 advice and oversight from Professor 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 sweet pea, whilst the head of the 

 largest specimen was the size of that of the Common Squirrel. To these 



