DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 155 



human embryos obtained are the results of spontaneous abortions, there is 

 reason to suspect that such embryos are not normal. To the physician, as 

 well as to the embryologist, it is important, therefore, that there should be some 

 criteria for differentiation between normal and abnormal or pathological 

 embryos. 



Gross anomalies, or monstrosities, such as cases in which the head or some 

 other member of the body is lacking, or in which the head is disproportionately 

 large or disproportionately small, or in which two embryos are directly united, 

 or in which the foetal membranes are partially lacking, or in which the mem- 

 branes are present and the embryo wholly or partially lacking, and many other 

 anomalous conditions, can, of course, be recognized at once. Extensive 

 pathological changes or processes of disintegration in the tissues of the em- 

 bryo or foetal membranes are also easily recognized. But there are many less 

 obvious anomalies and pathological conditions which, nevertheless, are im- 

 portant. Such cases are most difficult to differentiate. 



The fcetal membranes not infrequently are useful in determining whether 

 an embryo has followed the normal course of development. During the first 

 month the amnion invests the embryo rather closely when development is 

 normal. If the amniotic sac is disproportionately large, however, it is a mark 

 of abnormal or pathological changes. In some cases an amniotic sac 50 to 60 

 mm. in diameter contains an embryo but a few millimeters in length. In the 

 earlier stages of development, before the amnion enlarges sufficiently to reach 

 the chorion, there is present a delicate network of fibrils, the magma reticulare, 

 which is attached to both chorion and amnion and which serves as a sort of 

 anchor for the amnion. In abnormal or pathological cases the magma reticu- 

 lare may become wholly or partially fluid or granular, or may become greatly 

 increased in amount. It may even extend through the amnion and reach the 

 embryo itself. 



Normal human as well as other mammalian embryos in the fresh condition 

 are more or less transparent, and such structures as the heart, the larger blood 

 vessels, the liver, and the brain vesicles can be seen through the skin. If the 

 embryo has been dead for some time or has undergone pathological or degen- 

 erative changes, the transparency is lost. 



Where pathological or degenerative changes in the embryo or its membranes 

 are suspected but cannot be definitely determined by macroscopic examination, 

 recourse may be had to sectioning and staining. 



References for Further Study. 



VAX BEXEDEX, E.: Recherches sur les premiers stades du developpement du Murin 

 (Vespertilio murinus). Anat. Anz., Ed. XVI, 1899. 



BRYCE, T. H. and TEACHER, J. H.: AD Early Human Ovum Imbedded in the Decidua. 

 Mac Lehose & Sons, Glasgow, 1908. 



