28 DOUBLE MONSTROSITY— HEMIDIDYMUS 



(A) Seeing that material is drawn in considerable amount from the adjacent margin of the 

 blastoderm on either side to aid in the formation of the lateral parts of the body, any cause which 

 fixes this material or diminishes its mobility will induce a corresponding outward pull or drag on to 

 the two sides of the embryonic rudiment. This may well be strong enough to make the growing end 

 of the rudiment divide into two horns, each of which would then naturally proceed to grow backwards 

 and would attempt to become complete and bilateral in itself, according to a potentiality which seems 

 to be inherent in all embryonic apical systems. Completeness on the inner or adjacent sides would 

 not be attained, however, there being no material at hand to aid in the formation of the muscle 

 plates on these sides. Under normal conditions it is the gradual extension of the blastoderm 

 enclosing the yolk which continually brings the required marginal material into serviceable position, 

 and accordingly any condition which prevents the spreading of the blastoderm or increases the area 

 to be covered may cause the pull or drag referred to above. Hemididymus may therefore be 

 expected to appear (1) in ova the vitality of which has been lowered from any cause, e.g. hatching 

 under unfavourable conditions, as in agitated water, in water deficient in oxygen, or having foreign 

 substances in solution, and (2) in ova which have become partially swollen through differences of 

 osmotic pressure or other cause. It is a common occurrence in ova with lowered vitality that 

 definite organs are the first to suffer, whilst the undifferentiated superficial blastoderm, and later even 

 the mesenchyme, may continue to spread. Here we have a possible explanation of the fact 

 brought out by Lereboullet, Oellacher, and Eauber, that a thin epidermal layer bridges the 

 space between the diverging halves of the body, and also of the additional fact noted by Eauber 

 that the underlying yolk in this region contains nuclei without cell boundaries, e.g. that the primary 

 entoderm may be present although the definitive entoderm is never formed. 



The thickened ectodermal tubercle noted by Oellacher at the posterior angle of the cleft 

 between the two body-halves in mesodidymus is difficult of explanation, but may perhaps be due to 

 the crowding together of surface tissues which will naturally take place in the angle of re-union, and 

 which will be augmented afterwards through the supervention of secondary fusion. That the last 

 named process occurs in an appreciable degree is evidenced by the fact that the doubling of the 

 notochords extends very considerably further back than doubling of the spinal cords, or of the 

 alimentary canals. 



(B) The factors grouped under (B) have to do with postero-mesial obstruction or defect, and 

 this may be caused by mechanical injury to the yolk or to the tip of the growing tail bud. The 

 hemididymi produced by Kopsch {132) through electrical stimulation were no doubt of this kind. 

 Each of the divisions thus produced will attempt to complete itself as described under (A), but here 

 again the absence of a supply of lateral material on the inner or adjacent sides prevents the 

 realisation of the perfect bilateral condition. 



Alike in (A) and (B) the two axes will now naturally follow in their growth the thickened 

 margin of the blastoderm. If the extension of the latter has been greatly interfered with, the axes 

 may never again be brought together, and thus the katadidymous condition may result. Or if vitality 

 has been only slightly lowered, the axes may become approximated posteriorly, through delayed 

 completion of the natural process whereby the yolk is covered. The tail buds now becoming united 

 may give rise to a single posterior portion, and thus the mesodidymous type described by Lereboullet 

 and Oellacher will result. 



(C) Mention of the causes under this heading is made on chiefly theoretical grounds, since 

 observations on their nature and action are awanting. But there is every reason for believing that 

 the growing embryonic axis of a fish possesses that potentiality of fission which manifests itself from 

 time to time in the growing axes of other vertebrates, as in apical growth-systems generally. 



Eauber's " posterior radiation " theory attempts to express and formulate the potentiality 

 in question, which was admirably enunciated by Cleland (39), as a principle even wider in its 

 nature and application. 



Prevalent Types of Monstrosity in Fishes and in other Vertebrates. The chief difference between 

 fishes on the one hand and birds and mammals on the other, as regards the types of monstrosity 



