92 



THE BIOLOGY OF TWINS 



C-bV 



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- 



FiG. 35 



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. 



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 {DasypusY^ 



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 



W tmt 



Fig. 37 



Fig. 38 



Figs. 37 and 38. — Two views of a 

 double embryo of the ferret (after 

 Assheton). 



