DOUBLE MONSTROSITY— CAUSATION 5 



tionate numbers of monstrosities. So far as one can gather, this seems also in a general way to 

 be the experience of fish hatcheries, though full and careful records from these are not as yet 

 available. Von Baer's (-5), Oellacher's {17 G), and Rauber's {203) observations point to the same 

 conclusion, which may also help to explain the exceedingly wide variations in the other frequency 

 records (p. 1). Lereboullet farther notes that samples of the same brood of eggs tend to exhibit 

 similar types and numbers of monstrosities even under differences of environment. Indeed, the only 

 constant result which could be ascribed to the action of external factors was the production of defects 

 in development. It will be noted that this corresponds on the whole with the data afforded by 

 Dareste's {53) experiments on the eggs of birds. Evidence may also be adduced on the general 

 question from a consideration of the relative frequencies of double and triple monsters (see 

 note on p. 33). 



The likelihood cannot, of course, be excluded, that external factors sometimes induce the 

 production of double monstrosity in the developing eggs of fishes. As is well known, this has been 

 demo experimentally in the case of many invertebrate and of some vertebrate ova. The result 

 has usually been obtained through virtual or complete separation of individual cells or of cell masses 

 in the earlier segmentation stages. It will obviously be difficult to produce the requisite degree of 

 separation in typical meroblastic ova, and particularly in those of the Sahnonidae, where the 

 earliest divisions affect the nuclei alone, and the first stages in segmentation are syncytial ones. 1 

 The above circumstances may help to explain the common failure of experiments on twin production 

 in the eggs of osseous and cartilaginous 2 fishes as compared with the fruitful results of similar 

 experiments in the Amphibia, and to some extent also in the lamprey (Batadlon 12). This last 

 author's {14) apparent success in producing polyembryony in the eggs of Leuciscus, through differences 

 of osmotic pressure, should also be noted (see p. 30), but it does not by any means invalidate the 

 view that the ordinary occurrence of duplicity in fishes is germinal rather than environmental in its 

 origin. The same thing holds good regarding the occasional production of katadidymus under 

 experimental conditions (Lereboullet 14-3, Stockard 236, Kopsch 132). In fishes this condition 

 provides an entirely peculiar type of duplicity (p. 25). 



The production from a single ovum of twins or of twin embryos more or less united has long 

 been known to occur from time to time in all classes of vertebrate animals. Eecently polyembryony 

 has been shown to be the normal condition for the edentate mammal Tatusia? in which the seven 

 or eight young produced at a birth all develop from a single egg. 



The view has often been suggested that the blastoderm may be looked upon as a stock, 

 able to give rise vegetatively, so to speak, to more than one embryo. The natural comparisons 

 have been drawn between this faculty and the alternation of generations which occurs normally 

 in some groups of lower animals and in plants. It has even been sought to recognise alternation of 

 generations in the development of all animals. More probably, however, in animals, twinning, 

 double and multiple monstrosity, polyembryony, and alternation of generations, provide instances 

 in which a common " potentiality " has become realised, and beyond that are not necessarily 

 connected by any nexus of a direct or phylogenetic character. 



'Kopsch (F.) in Arch. Mikr. Anat. 78, 1911 (618-659). 



2 Recently Eismond (G3a) has given an interesting account of experiments on the eggs of Raia clavala and Raia alba 

 (luring the early segmentation stages. He finds that separated portions of a blastoderm often show remarkable activity 

 in reuniting to form a whole, which may then proceed to normal development. On the other hand, the separated portions 

 may remain apart or even undergo further spontaneous division, and a number of embryonic rudiments may arise in the 

 complex thus produced. The number of rudiments in one particular case was four. Two of these were found on one 

 of the fragments ; the other two occurred each on a single fragment, whilst two of the fragments provided none. Later, 

 the clefts between the fragments disappeared completely, and the blastoderm, now single to all appearance, proceeded 

 to extend over the yolk. Two of the rudiments were, to begin with, at an angular distance of about 55° from one another 

 on the margin of the blastoderm. A day later these rudiments were found to have become closely approximated. They 

 were, however, no longer normal in appearance, but seemed to be in process of degeneration, and aceordinglj' it was 

 judged best to fix and preserve the whole specimen at this stage. 



3 Fernandez (M.), " Beitrage zur Embryologie der Gurteltiere," Morph. Jahrb., Leipzig, 30 (302-333). 



