CHAPTER III. 



CYCLOPIA. 



PI. XXIV. fig. 100 (external appearance, see also Pis. I. fig. 3 and II. fig. 11); XXIV. figs. 101- 

 104 (various structures); XXV. figs. 107-108 (sections of head), and figs. 109-110 (semicyclopia). 



Classification and structure, - - - - p. 40 



Summary and comparison, ... - - - p. 42 



Other records in fishes, ... - p. 43 



Causation, - ... - - p. 44 



Cyclopia rarely manifests itself in developing trout eggs. Under ordinary fish-hatchery con- 

 ditions it may be put down as being ten or fifteen times less frequent than double monstrosity. 

 Altogether I have obtained only six examples. It is noteworthy that two of these were provided by 

 double monsters in which one or other of the twin heads showed the defect in question (PI. I. fig. 11, 

 cf. I. fig. 3). 



The total number of specimens is hardly large enough for systematic purposes, but I find that 

 here, as in the first groups of double monsters, the condition of the nervous system affords a basis 

 for classification into at least two main divisions. The first (A) is characterised by fusion, more or 

 less complete, of the cerebral lobes (two specimens) ; while the second (B), in addition to fusion of 

 the cerebral lobes exhibits fusion on the part of certain structures belonging to the third ventricle 

 and the mid-brain regions (three specimens). The sixth specimen was not cut into serial sections, and 

 accordingly I cannot say for certain to which division it belongs. In view of the probable causation 

 of cyclopia (p. 44) it will be understood that while the condition is always associated with cerebral 

 defects, these defects need not be of a perfectly uniform character, nor need the fusion or reduction 

 of the two eyes always correspond in amount with the fusion or reduction present in the two sides 

 of the brain. 



(A) Cyclopia with Fusion of the Cerebral Lobes (two specimens). 



The external appearance is illustrated by PI. XXIV. fig. 100. The front of the head is wedge- 

 shaped, its size being reduced in the transverse and increased in the vertical line. The large median 

 eye is overarched by a mesial frontal process carrying a pair of small closely approximated olfactory 

 pits (Fig. 101). Upper and lower jaw arches are present. The posterior part of the head and the 

 body are normal. 



Cranial Skeleton. The skeleton is greatly modified in front of the pituitary region. The 

 trabeculae cranii pass downwards so as to lie below the median eye. They are widely separated from 

 the base of the brain and they take no part in the formation of an olfactory capsular cartilage. 

 Anteriorly, they articulate with short palato-quadrate bars. In the normal Trout embryo at a 

 corresponding stage, the trabeculae, though united, still show evidence of their double origin. But 

 in all my cyclopic specimens the trabeculae form an absolutely single piece right back to the 

 pituitary space. 



A rudimentary olfactory capsule is derived from the united anterior ends of the supra-orbital 

 bars. This united portion lies in the frontal process and is perforated by the two small olfactory 



