No. 2.] EMBRYOLOGY OF THE LIZARD. 351 



suppose that it appeared at one time in one smaller group, from 

 which it has been inherited. The same is true of the segmenta- 

 tion of the body, and is illustrated in the incipient rudimentary- 

 segmentation shown by Hubrccht to exist in the Nemcrtina. 

 The aggregation of a yolk-mass is an embryonic feature, and 

 appears much modified in closely allied forms. We must sup- 

 pose that the character of a type is due to selection acting 

 chiefly on older individuals, at least after the period when a 

 yolk-mass is present. When the character of the type has been 

 determined, the nutritious matter of the embryo seems simply 

 to collect at that spot where it interferes least with the develop- 

 ment of the important organs. In view of the variability of the 

 yolk-mass in the vertebrata and annulosa, there seems to be no 

 reason for supposing a connection in this regard between the 

 two groups ; no more than for supposing a connection between 

 the so-called placenta of Salpa and the placenta of Mammals. 

 While the above objections to the annelid-inversion theory 

 have merely a negative force, there are certain positive objec- 

 tions, which are met with great difficulty. The first of these 

 relates to the homologizing of the vertebrate brain with the 

 oesophageal nerve-ring, and the supposition that the oesophagus 

 once passed through the vertebrate brain. At one time this 

 objection appeared to be met with the aid of the pituitary body 

 and the pineal gland, but more recent researches have closed 

 this means of escape and failed to open plausibly any other. 

 The connection of epiblast and notochord ventral to the brain, 

 v/hich I have described, adds new strength to the objection, and 

 makes it seem insurmountable. The second objection relates 

 to the inversion of position. We cannot suppose here a lateral 

 turning over after the gradual manner in which it is supposed 

 that in certain forms of the echinodermata the vertical axis 

 gradually became the longitudinal horizontal axis, and of wdiich 

 intermediate forms remain. The supposition that the lateral 

 sides of the worm became dorsal and ventral leads to the con- 

 ception of a form that is incompatible with our knowledge of 

 the symmetry of animals and the locative relation of their 

 paired organs to the line of the force of gravity. On the other 

 hand, if we suppose the change to have been sudden, we meet 

 with equal difficulty in conceiving a form of such pecuHar in- 

 stability that it could at once turn itself upside down, and place, 



