92 



THE BIOLOGY OF TWINS 



Fig. 3S 



Fig. 36 



Figs. 35 and 36. — ^Two views of sheep 

 ovum with twin embryo (after Assheton). 

 It is unlikely that such double embryos 

 develop very far. 



constructive theory of polyembryony in Tatusia (Dasy- 

 pus) is then offered, which is based upon the unique 

 combination of three conditions: 



(i) the development of the blastocyst within the central lumen 

 of the uterus which has allowed of a considerable expansion of 



the ectodermic plate, 

 owing to the rolling 

 up of the blastocyst 

 cavity as in Lupus 

 now; (2) 'inversion of 

 layers' by which the 

 ectoderm plate be- 

 comes invaginated 

 into the large cavity 

 of the blastocyst sub- 

 sequent to its ex- 

 pansion; (3) a late 

 formation of a thick- 

 ened mass of trophoblast over the entire expanded plate putting 

 more pressure on the center than on the periphery of the ecto- 

 dermal disk. 



Assheton points out 

 that "if we take a case 

 like that of Lupus and 

 superimpose upon it 

 the Trager of Mus we 

 should get a condition 

 which would approxi- 

 mately be that of 

 Tatusia [Dasypus]." 



This purely morphological explanation of what 

 seems to me unquestionably a physiological process 

 serves only to obscure the real problem of the causal 

 basis of polyembryony. I am, however, in agreement 



Fig. 37 



Fig. 38 



Figs. 37 and 38. — Two views of a 

 double embryo of the ferret (after 

 Assheton). 



