458 M ERISTIC VARIATION. [PART I. 



It was intruded to liavc introduced here some account of the 

 s and very rare cases in which, for a greater or less region of 



-|iinc, r"iT< -]H>iidiii'_: halt- vertebra?, on either side of the middle 

 line. an- not united together in their proper order, but I fear this 

 would be too great a digression. For references on the subject see 

 LKVKI.IM;, ul,s. anat. rarior., Norimb., 17iS7, Fsc. 1, cap. in. p. 145, 

 Tab. v.: SANDIFOKT, J/</.s\ anat, Leyden, 1835, iv. p. 74. PI. 

 n. xxvin. ; REID 1 , Jmir. of Anat., 1887, xxi. p. 7b', Jig. : Guy's 

 Hop. Hep., 1883, p. 132. 



(>K Ai:SK\( K OF DIVISION IX THE MIDDLE LlXE. 



This phenomenon is the converse of that described above. 

 Kxamples of median union are found in many organs of different 

 kinds. In vertebrates such union is especially well known in the 

 case of the eyes, the ears, and the posterior limbs, producing the 

 cyclopic, synotic and symmelian conditions respectively. 



Each of these is of some interest to the student of Variation by 

 reason of the symmetry and perfection with which the union takes 

 place. In the cyc-lopi.m the degree to which the two eyes are com- 

 pounded presents all shades intermediate between the perfect duplicity 

 of the normal and the state in which the eye-balls are united in the 

 middle line of the forehead and have one circular cornea 2 . These 

 variations are closely comparable with those of the eye-spots on feathers 

 referred to on p. 449 ; for there also all stages are seen between a pair 

 of eye-spots placed one on either side of a middle line and complete 

 union to form one eye-spot bisected by the middle line. There is of 

 course no normal vertebrate having the eyes thus united in the middle 

 line, but as MECKEL has remarked, the case of the cyclopian is not 

 essentially different from that of the Cladocera in which the compound 

 eyes, paired in other Crustacea, are united to form a single median eye. 

 The cases No. 718 and 719 of median union of the compound eyes of 

 Bees may also be considered in this connexion. 



A very similar series of variations occurs in regard to the ears of 

 vertebrates, which in the synotic or cephalotic condition are compounded 

 in the middle line to a varying degree 3 . Such union of the ears is 

 especially common in the Sheep, cyclopia being most frequent in the 

 1'iir. I > \i:r:sTi-: 4 states that the first beginning of the cyclopian condi- 

 tion appears in the Chick as a precocious union of the medullary folds 

 in the region of the fore-brain, occurring before the optic vesicles are 

 fully formed from it. The degree to whu-h the union of the eyes is 

 complete then depends on the earliness with which the folds begin to 

 meet relatively to the time of budding off of the optic vesicles. DARESTE S 

 also declares that the cephalotic state is similarly first indicated by a 

 premature union of the folds in the region of the medulla, taking place 



1 A ease in Man, resembling No. 7. 



For an extensive collection of cases illustrating the various degrees of cyclopia 

 sec especially AIILFKLH, Mi.mh. <l. .!// nxcli., Abschn. n. Iss-j. 



5 For figures see e.f/., OTTO, Mus. anat. path. I'rntisl., PI. i. fig. 5, PI. in. fig. 2 

 (Lambs); Gri:ia>AN, Mounts, f. Gebitrt.il;.. \. p. 170, PI. i. (Man) and many more. 



4 Compti'K n-ndn*, 1*77, i.xxxiv. p. 1(138. 



f> /. c., 1880, xc. p. 191. 



