474 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



wall destined to give rise to tlie body of the embryo, in which is 

 therefore concentrated the greatest developmental activity, is hindered 

 in the increase in area which would be the normal result of its growth 

 and in consequence it has found accommodation by bulging inwards and 

 giving rise to the inner mass. 



If this is a true explanation of the nature of the inner mass we should 

 expect that this might become apparent through the inner mass opening 

 out and resuming its primitive condition as a part of the blastocyst wall 

 in the event of the restrictions upon its extension becoming lessened in 

 later stages of development. This actually happens and some of the 

 most striking peculiarities in the early development of particular types 

 of mammal are due to the great differences in the length of the period 



c.i.'ni. 



C-im. 



Diagrammatic sections tlirough blastocyst of Tupaia (A), Mouse (B), and Man (C). (A, after 

 Hubrecht ; C, after Bryce.) i;, Coelome (precociously developed); c.i.m, cavity of inner mass; 

 y.s, cavity of yolk-sac. ' 



during which the expansion of the portion of blastocyst wall under 

 discussion is restricted. 



In a relatively primitive insectivorous mammal Tupaia a cavity 

 develops in the ectodermal portion of the inner mass at a very early stage 

 (Fig. 199 A, c.i.m.), and this presently comes to open to the exterior and 

 the whole inner mass opens out so as to form simply a part of the 

 blastocyst wall in the manner we should expect. In the Rabbit the same 

 condition is attained at an early period, though here there is no trace 

 of the " opening out " process, the mass simply spreading outwards into 

 a thin layer. In a Vole (Arvicola) as in Tupaia a space makes its 

 appearance in the interior of the inner mass but the flattening out 

 process is delayed until a period after the amniotic folds have made 

 their appearance. The meeting of these folds isolates the amniotic 



