AXD AFFINITIES OF TARSIUS. 479 



representatives of the stock fi-oiii which the higher Primates 

 originated. 



Turning now to Tarsitis itself, our knowledge of its deve- 

 lopment may be said to be fairly complete, thanks to the 

 untiring labours of that illustrious embryologist, the late 

 Professor Hubrecht. In a series of papers he has provided <.is 

 witli richly illustrated accounts of its early development, its 

 f<ctal membranes and placentation, and he has discussed its 

 attinities at some length. As the result of his investigations, 

 Hubrecht came to the conclusion that Tarskis is no Lemur, 

 but is more nearly related to the Anthropoidea and should be 

 classified with them. 



And there can be no doubt i\.t -all that in many features 

 of its development Tarsius does appear to be more closely 

 related to the Antliropoids than to the Lemuroids. That much 

 is obvious from the data set forth in the table, but there are 

 also certain dift'ei'ences to be noted, a,nd our task is to try and 

 evaluate tliese resemblances and differences in terms of affinity. 



The principal developmental features in which Tarsius differs 

 from the Lemuroids and agrees with the Anthropoids may be 

 recapitulated as follows: — (1) the early attachment of the 

 blastocyst to the uterine lining through the proliferative activity 

 of the trophoblast ; (2) the precocious formation of the extra- 

 embryonal mesoderm and c<elom and the correlated early 

 separ:(tif)n of the yolk-sac as a small reduced structure ; (3) the 

 functional replacement of the vesicular allantois by the so-called 

 connecting or ventral stalk : and (4) the formation of a massive 

 deciduate placenta in which the maternal blood circulates through 

 lacunar spaces in the syncytial trophoblast. 



Set down in this bald fashion, these striking resemblances 

 between Tarsius and the Anthropoids would seem to settle 

 the question of its affinities without more ado ; but let us 

 examine them a little more closely. 



The attachment of the Tarsius blastocyst to the uterine wall, 

 at a time when it measures only 0-3 mm. in diameter, is effected 

 by the activity of the trophoblast over a localised patch situated 

 immediately behind the posterior margin of the embryonal area 

 (text-fig. 2, 2^i-tr.). All that we can say in regard to this attach- 

 ment is that it represents a definite advance of an adaptive nature 

 on the Lemuroid condition. It is the necessary preliminary to the 

 formation of the discoidal deciduate placenta., and it is a point 

 of interest that the attachment is effected immediately behind 

 the embryonal area and so in proximity to the region where 

 the allantois normally develops, although later on the definitive 

 placenta, as the I'esult of differential growth in the fcetal 

 membranes, comes to lie opposite the embryo. And it may 

 also be regarded as the first step in the evolution of the relations 

 of the Anthi'opoid blastocyst to the uterus, which range from the 

 same primitive central type of development that is seen in 

 Tarsius, with in some cases an additional second attachment 



