480 



ON THE ZOOLOGICAL POSITION 



to the uterine wall (as in the Old- and New- World Monkeys), to 

 the interstitial naode of development in which the blastocyst, 

 w^hilst still quite minute, burrows its way, througli the agency 

 of its trophoblastic covering, into the uterine decidua and so 

 becomes completely imbedded, its trophoblast proliferating over 

 its entire extent (as in Man and the Anthropoid Apes). Thus 

 the relatively simple attachment in Tarsius points the way 

 to the much more highly specialised Anthropoid condition. 



Then, in respect of the very early formation of the extra- 

 embryonal mesoderm and ccelom, Tarsius exhibits marked 

 adaptive specialisation as compared with the Lemuroids, 

 and this same specialisation again I'eaches its acme in the 

 Anthrojooids. 



Text -figure 2. 



fBrnhr.. 

 ecto. 



ex 



Tarsius spectrum. Diagram (after llubrccbt) to show the structure uf the early 

 blastocyst, shortly after attachment to the uterine wall. Note the connecting 

 stalk {est.) already present in the form of a band of mesoderm extending from 

 the posterior end of the ectoderm of the embryonal shield (emhr.ecto.) to the 

 margin of the attached area of placental trophoblast (pl.tr.), the yolk-sac 

 {i/-s.c.) not yet fiec and the extensive extra-embryonal ctvlom (f.i-.cce.). 

 eft. chorion. 



Whereas in the Lemuroids, the extiu- embryonal mesoderm 

 would appear to be formed, like that of the lower Mammals-, 

 simply by tlie gradual peripheral extension of the embiyonal 

 into the bilaminar wall of the blastocyst, here, in Tarsias, 

 it is formed precociously, long before the embrj'-onal mesoderm 

 has made its appearance, as a cellular mass proliferated, in 



