182 EDWARD MCCRADY, JR. 



The young thus orients itself away from gravity without 

 the assistance of a righting reflex or of any consciousness 

 of 'up or down.' This is a true tropism, or simple, automatic 

 orientation, conditioned solely by the structure of the body 

 at the time the functional forelegs and the non-functional 

 hind legs. 



The mother sits in such a position that the path away from 

 gravity leads to the pouch. And as a result the young 

 wanders blindly into the place of safety and food. Probably 

 many of them miss it, for the young found in the pouch are 

 nearly always fewer than the embryos found in the uteri 

 shortly before birth. 



The crawling movements have been studied in some detail. 

 The youngest foetuses which I have tested for reflexes were 

 11 days and 9 hours after copulation (34 hours before their 

 litter mates from the other uterus were found in the pouch). 

 These stage 33 foetuses were placed in mammalian Ringer's 

 solution. Their vascular system was intact no embryonic 

 or extra-embryonic vessels either ligated or cut. Their hearts 

 could be seen beating. No foetal respiratory movements were 

 observed. Hairs and fine gut sutures (medium dermis suture 

 size 000) were used as probes. No reflexes could be evoked 

 by tactile stimuli on any part of the body. Unfortunately, 

 only superficial stimulation was tried, so I am unable to say 

 whether the muscles can respond to deep, direct stimulation, 

 but this is not likely, as not even fixation in Bouin's fluid will 

 cause any muscular contractions at this time. The most 

 serious objection to this experiment is that the Ringer's solu- 

 tion was not artificially heated. The temperature of the room 

 and of the solution at the time was probably about 85F. 



Late stage 34 foetuses, on the other hand, if removed from 

 the uterus and freed from their membranes, execute perfect 

 crawling movements like those of the newborn to be described 

 next. So the development of this complicated behavior is very 

 rapid the non-motile to the crawling stages in 24 hours or 

 less. 



