The ]\Iorphology of Cosniobia ■i"21 



incomplete separation, of anlagen extends also to the eye-muscles, so 

 that this case represents another stage in the same cosmobiot.ic series 

 with the previous one, thus making the comparison one of extreme 

 interest. 



In one particular the specimen was deformed ; the brain was everted 

 and the cranial cavity was very small. This, or some similar defect of 

 the brain, as is well known, is quite common in certain types of abnormal 

 cosmobia, and may be either primarily or secondarily connected with the 

 same cause that produced the cosmobiotic abnormality, but the fact that 

 these defects are by no means constantly associated with the latter is 

 strongly indicative that they result rather from a secondary, probably 

 mechanical, cause than from anything inherent in the germ and thus 

 predetermined from the beginning. It would thus be likely that the 

 early embryo in such cases would be free from these defects and that 

 their appearance would date only from the point at which some mechan- 

 ical barrier to normal development would be formed. For example, 

 the double pig embryo just described shows no defect, and the sections 

 appear as in normal animals, but it is easy to conceive that at a little 

 later date, when the parts became more complex, the development w':uld 

 not have continued so harmoniously and that thus deformation of parts 

 would be inaugurated. Since also the mechanical problems of normal 

 development differ in different animal groups, it is likely that an abnor- 

 mal, though cosmobiotic, form, which in the case of one animal usually 

 develops as far as birth without deformation, may, in the case of another 

 species, meet some difficulty in the mechanical adjustment of parts 

 which may modify or even prohibit development beyond a given stage. 

 This would furnish an explanation of the well-known fact that certain 

 types of cosmobiotic monsters are characteristic of certain groups of 

 animals ; and in this connection it is worthy of remark that in birds, the 

 one group in which we do happen to know a little about embryos of 

 cosmobia, the types most frequently observed during early embryonic 

 life are not the ones characteristic of this class of animals after hatching. 

 Thus varying grades of Janus monsters are often found in embryos of 

 the first two or three days, but records of this t^^pe among birds after 

 hatching must be extremely rare, and I cannot recall a single instance, 

 either in literature or in my own experience. 



From the study of such embryos it would seem that the mechanical 

 moment which here introduces a complexity that proves disastrous is the 

 turning of the head to the right, characteristic of early bird embryos. 



