DEVELOPMENT IN ECHIDNA 



21 



cutis part of the primordia is well marked owing 

 to its condensed character and the presence of 

 numerous capillaries (Figs. 8a and 8b). The 

 general topographic features, especially those 

 of the skin muscle, remain unaltered. 



Whilst the primary -primordia long persist in 

 this condition, soon after birth other differen- 



FiG. 7c. — ^Echidna: Transverse Section or Semon's 

 Embryo, showing the Primary-Primokdia when 

 IT FORMS A Lens-shaped Projection on the 

 Surface. 



tiations begin, leading to the formation of the 

 incubatorium. Although this develops quite 

 independently of the primary-primordia, their 

 existence is closely connected with the causes 

 of the formation of the pouch. 



Immediately after the hatching of the young, 

 the skin closes over the navel opening. But 

 the skin-muscle does not grow, because the 



