THE EMBRYO OF A GIBBON IN THE THIRD STAGE. 131 



consequence of the destruction of the maternal tissues the walls of some of the 

 blood-vessels have been broken through, and this has allowed the blood to escape 

 from those vessels into the lacunae of the trophoderm. 



The trophoderm of the ovum offers a very complex picture, owing chiefly to 

 the changes which it is undergoing. The changes are due apparently to hyper- 

 trophic degeneration. The layer of the chorionic ectoderm next to the mesoderm 

 retains more or less evidently a cellular character. The remaining portions tend to 

 form a syncytium in which the nuclei become enlarged and the cell-boundaries 

 obliterated, while the protoplasm of the cells also changes in character and becomes 

 more homogeneous in texture and much denser. The syncytium disappears by re- 

 sorption, and its disappearance causes the formation of spaces in the trophoderm. 

 Many different pictures occur in connection with these processes, for in some places 

 the nuclei tend to gather in groups, in others they disappear; in some instances 

 strands of degenerative material are left, while nearby some of the trophoderm may 

 retain its more primitive appearance and be but slightly altered. Finally, it should 

 be noted that at various points the chorionic mesoderm is growing out into the 

 trophoderm. Each of these mesodermic outgrowths is to be interpreted as the 

 anlage of the central portion of a chorionic villus, and out of the neighboring 

 chorionic ectoderm will be differentiated the ectodermal covering of the villus. It 

 seems, from a comparison of later stages, that the trophoderm degeneration never 

 goes so far as to leave any of the chorionic villi without an ectodermal covering. 

 But this covering varies extremely in its exact character as we find it in later 

 stages, even in adjacent parts of the same villus, for it may be either a single layer 

 of cells or a layer of cells covered by a thin coat of syncytium or merely a syncytial 

 layer (compare page 354). The disappearance of all of the trophoderm, except 

 so much as remains to share in forming the ectodermal covering 'of the villi, pro- 

 duces the so-called intervillous spaces of later stages, in which, as above stated, 

 maternal blood circulates. 



An ovum in situ slightly more advanced than Peters's has been described care- 

 fully by Herzog. The specimen is now in the Harvard Embryological Collection, 

 Series 1500. It differs from Peters's ovum in having a prolongation of the entoderm 

 into the body-stalk to make the anlage of the allantois. 



The Embryo of a Gibbon in the Third Stage. 



The embryo to be described was obtained from a female Hylobaies concolor in 

 Borneo by Selenka. It is a little more advanced than Frassi's human embryo, men- 

 tioned on page 119. It still had traces of the primitive streak, at the anterior end 

 of which was an open neurenteric canal. The medullary plate was partially differen- 

 tiated from the embryonic shield. It was thoroughly studied by Selenka, and is 

 one of the best known very early ova of any primate. The entire ovum is repre- 

 sented in figure 75. The figure was reconstructed from the sections. It shows the 

 chorionic membrane studded with villi. The diameter of the chorion was about 8.5 



