60 PATHOLOGICAL CONDITIONS OF OVUM, ETC. 



my Class I. (p. 3) as showing duplicity in a still less marked degree. Meantime we do not 

 need to do more than call attention to this possibility, pointing out in its favour that Barbieri's (7) 

 early double trout embryo exhibited much greater doubling in the floor than in the roof of the 

 brain, while Lereboullet (see p. 11) showed in general that a very considerable degree of early 

 axial duplicity can afterwards become completely or almost completely lost, through the working 

 of secondary fusion. Schmitt's Group 7 makes provision for some such Class. At the same 

 time, the little that is known on the subject, renders the alternative possible, that obscure 

 and limited cases of axial duplicity may have had their origin in union of the longitudinal or 

 parallel type (p. 29). 



The second specimen I shall describe under the heading of Eeduction was quite normal in 

 appearance, except for the presence of a tiny refractive knob behind the left eye. Examination 

 of serial sections shows the knob to contain a lens of considerable size, enveloped in muscle-fibre, 

 but unaccompanied by any other eye-structure, and lying in front of an exceedingly minute fore- 

 brain and third ventricle. The cavity of this third ventricle communicates with the mid-brain cavity 

 of the normal head (PL XXIV. fig. 106). The embryo was quite lively when obtained, and its 

 chances of survival would probably not have been appreciably diminished by the small tumour 

 in question. It will be seen from fig. 2 that the functional eyes and fore-brain belong to a 

 predominant right twin head, as also do the olfactory organs, the mouth, and the anterior cranial 

 cartilages generally. The back part of the brain and the whole of the body are, however, composite, 

 since their left moiety represents structures which are continuous with the left side of the 

 left (aborted) twin, while their right side is a continuation backwards of the 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. 



PATHOLOGICAL CONDITIONS OF THE OVUM OK EAELY EMBKYO. 



Some of these have been referred to already, for example under the last two of Lereboullet 's 

 types (p. 10). Numerous single instances may be found throughout this author's account of his 

 observations. Attention should also be drawn to Kauber's description of the following anomalous 

 conditions in the development of pike and salmonid eggs (3025 (668-702)). 



(a) Examples (salmonid) just at the end of segmentation, of blastoderms showing irregularities 

 of outline or surface, and want of uniformity in the size of the segmentation cells. 



(5) Instances (salmonid) in which the margin of the blastoderm has failed completely, or almost 

 completely, to become thickened in order to give rise to the embryo-rudiment. In such -cases the 

 blastoderm may extend as far down as the equator or even the lower pole of the egg. The growing 

 margin tends to become irregular, showing projecting angles and lobes. 



(c) Defective formation of the anterior, middle, or posterior portions of the embryo. Five 

 examples of salmonid eggs are given in which the anterior part of the embryo is defective ; three of 

 the pike in which the anterior and middle portions are wanting ; one (salmonid) in which the 

 middle and posterior portions are defective, and another salmonid in which the embryonic rudiment 

 has not been formed at all. 



(d) Partial or total dehiscence of the body halves. This condition is referred to under 

 Mesodidymus (p. 25). 



(e) Oblique position of the embryonic rudiment. Several examples were noted in the salmon. 



Girdwoyn (81) makes reference to embryos without eyes, with both eyes present but rudi- 

 mentary, with only one eye and that in its normal place, with well developed head but rest of body 

 rudimentary, without head but with developed body, etc. Loeb (14&) describes fish embryos without 

 circulation, while Stockard (1236) and others refer to various pathological conditions produced in the 

 course of their experiments. 



