ORIGIN OF INCUBATOEIUM 25 



of the incubatorium is less resistant than the 

 rest of the abdominal wall. 



Through this tension only the caudal part of 

 the incubatorial area becomes depressed at 

 ffrst,. while the cranial portion, containing the 

 primary-primordia, remains quite unchanged. 

 It is not until the depression gradually extends 

 in the cranial direction that the primary- 

 primordia are finally drawn into the incuba- 

 torium (Fig. 10). The latter is then bounded by 

 the bundles of skin-muscle surrounding the 

 original incubatorial area and crossing each 

 other in front and behind it. This fact justifies 

 their being regarded as forming a sphincter 

 incubatorii, although they are not anatomically 

 distinct. Sometimes the navel-scar, which as a 

 rule quickly disappears, divides the incuba- 

 torium into two halves for a short time (Fig. 9). 

 But, of course, as Ruge has already remarked, 

 this is not to be taken as a proof of a paired 

 origin of the incubatorium. 



In these last stages the primary-primordia be- 

 come once again easily observable, since they 

 remain entirely free from hairs, which else- 

 where develop uniformly all over the skin 

 (Fig. 10). The consequence is that, in surface 



