1906.] ABNORMAL TROUT EMBRYOS. 451 



right side of the right twin. This gives an even more complex 

 mixture of " individualities " than is found in ordinary cases of 

 symmetrical double monstrosity. 



In this case of aborted twin head the lens alone of all the 

 eye-structui'es has survived. This is by no means infrequent, 

 even in cases of atrophy of the head uncomplicated by duplicity. 

 A good example is shown in fig. 3, which illustrates a transveise 

 section of a single atrophic head. The mouth, the lower jaw, the 

 trabeculaj, and the palato-quadrates are absent. One large lens, 

 clothed with muscle-fibres, is present on the right side ventrally 

 and compresses the lower part of the brain. A second smaller 

 hour glass-shaped lens lies beside it, all other ocular structures 

 being deficient (Plate XXXIII. fig. 3). 



(B) Local Deficiency or ReduqMcation of the Notochord 

 in Trout Embryos. 



While examining a numi)er of Trout embryos in sei'ial section 

 I came aci-oss three cases of local reduplication of the notochord. 

 In two of them the notocLord is bifid at its anterior extremity, 

 becoming single while still in the intra-cranial region. The para- 

 choi'dal cai-tilages are bi"oad in front and enclose both ends of the 

 notochord. There is no duplicity of any other structure. It is 

 perhaps remarkable that one of these embryos was a cyclops of 

 type B. 



The third example of reduplication of the notochord was found 

 in a set of sections which had been cut from an apparently normal 

 embryo for the purpose of serving as a typical series. In the 

 middle abdominal region the notochord is observed to divide into 

 two limbs which lie adjacent to, but quite separate from, one 

 another for four or five segments, and then unite again. Where 

 they are widest apai-t each has a separate sheath and separate sets 

 of neural and haemal arch cartilages. The adjacent cartilages are 

 disposed, exactly as in double monstrosities, at the region of 

 ti-ansition from the double to the single condition. These cases 

 seem to be examples of local fission afi'ecting a single axial organ, 

 rather than examples of ti-ue axial duplicity. 



Local deficiency of the notochord occurred in one specimen. 

 Here the notochord, which is noi-mal in the cranial and cervical 

 regions, ceases abruptly just behind the level of the pectoral fins. 

 After being absent for six somites, it reappeai-s and runs back- 

 wards normally along the rest of the trunk. Plate XXXIII. fig. 4 

 illustrates the appearance of a transverse section in the defective 

 region. The neural and haemal arch cartilages have fused 

 together to form a series of half-rings below the cord. Ventral 

 to these the lateral muscle-masses meet one another in a mesial 

 raphe above the dorsal aorta, forming a strong support and sling 

 for the veitebral column and the cord. 



Prog. Zool. Soc— 1906, Vol. I. No. XXX. 30 



