482 ox THE ZOOLOGICAL POSITIOX 



becomes hollowed out by the appearance in its inteviox' of a 

 cavity, the extva-embryonal ca^lom. The entodermal yolk-sac 

 earlier established only partially fills the space enclosed by the 

 ti'ophoblast, being in contact with the latter only in front (text- 

 fig. 2, y-s.c.). The ccelom now rapidly expands so as to fill this 

 space, and it also extends forwards into the front wall of the 

 blastocyst, separating the j^olk-sac entoderm from the trophoblast, 

 with the result that the yolk-sac becomes provided with an 

 independent wall of its own and projects into the crelom as a, 

 small free vesicle, whilst the chorion is completed as a continuous 

 membrane, which forms the outer wall of the embryonal for- 

 mation (text-figs. 3 & 4, y-s.c. & ch.). 



Thus in the mode of development of the extra-embryonal 

 mesoderm and coelom, in the precocious formation of the yolk- 

 sac, and in the early differentiation of the chorion, Tarsius is 

 much more specialised than the Lemui-oid — nevertheless, it still 

 retains in its ontogeny evident traces of the ancestral mode of 

 development of these structures. 



Tlie proximal part of the extra-embiyonal mesoderm in Tarsius, 

 referred to above, into which the extra-embryonal ccelom does not 

 extend, persists in the form of a solid, short, axial strand which 

 directly connects the hinder margin of the embryonal ectoderm 

 with the region of the chorion over which the placental tro- 

 phoblastic attachment has already been effected. This strand, 

 Hubrecht regards as the primordimn of the ventral or connecting- 

 stalk, the significance of which we shall presently discuss. 



Coming now to the Antliropoids, although we know com- 

 paratively little of the <:letails of their early development, it is 

 quite clear from our knowledge of the structure of the early 

 blastocyst tha,t their early ontogeny is much more specialised 

 than that of Tarsias. 



The earliest-known blastocysts are already either attached to, 

 or actually embedded in, the uterine decidua, the trophoblast has 

 proliferated to form a. syncytial network, into the meshes of which 

 maternal blood has penetrated. Inside the trophoblastic wall 

 there is alreadj- present a layer of extra-em In yonal mesoderm 

 (/. e., the chorion is established). This layer thickens at the upper 

 pole to enclose the embryonal primordium i^i'oper, in the form 

 of two closed vesicles — an upper, the amnio-embrj^onal vesicle, 

 and a lower, the entodermal yolk-sac. Here the embryonal 

 ectoderm which forms the floor of the amnio-embryonal vesicle 

 never becomes exposed on the surface as it does in Tarsius, and 

 the cavity of the vesicle, the primitive amniotic cavity, persists 

 to form the cavity of the definitive amnion, the amnion ai'ising 

 by the closed method and not by the closing in of folds as in 

 Tarsitis and the Lemuroids. The entodermal yolk-sac is most 

 precociously difierentiated as a small closed vesicle, and appa- 

 rently from the first lies remote from the trophoblast. The 

 extra-embryonal mesoderm is also most precociously developed, 

 but as to its mode of origin we have no knowledge. It is present 



