162 EDWARD MCCRADY, JR. 



with the discussion of the extra-embryonic membranes in 

 stage 29. But as it reaches it peak of development in stage 33, 

 a few more words about it are necessary at this point. 



Figure 49, C, shows how large the allantois becomes at its 

 maximum. In the specimen here photographed the chorion 

 and amnion have been dissected away. The frontispiece and 

 also figure 52 show the relations of the allantois to these other 

 extra-embryonic membranes. Looking at the frontispiece it 

 is easy to visualize the complete vesicle, and to see that the 

 embryo proper and the allantois are in a pocket hanging into 

 and completely surrounded by the endodermal yolk sac, which 

 lines the inside of the entire vesicle. 



In this position deep inside the yolk sac the allantoic vessels 

 cannot serve in a respiratory capacity. This function is left 

 entirely to the vitelline vessels, which are permanently super- 

 ficial. Accordingly, the allantoic vessels do not develop be- 

 yound the extent necessary to serve the immediate needs 

 of the thin-walled allantois itself. As the mesonephroi have 

 been functional for at least 24 hours (probably as early as 

 stage 30), and as no mechanism is provided for the transfer of 

 the urine to the maternal blood, the urine has to pile up in the 

 allantois, which correspondingly enlarges to accommodate it. 



Finally, in stage 34 the cloacal membrane breaks through 

 and releases urine into the amniotic cavity. From this time 

 on, the allantois looks collapsed as if it had expelled a con- 

 siderable portion of its contents back through the urachus 

 and cloaca and out into the amniotic cavity. And in har- 

 mony with this interpretation the amnion in stage 34 appears 

 rather bloated. These changes occur between 12 and 24 hours 

 before birth. 



Miscellaneous details. The lateral palatine processes are 

 forming in the mouth on either side of the tongue. The 

 caecum and small intestine still protrude through the um- 

 bilicus. 



All cranial nerves reach the blastemata of their end organs. 

 The third, fourth, fifth and sixth lumbar, and the first and 

 second sacral nerves develop intercommunications which 

 represent the beginning of the lumbosacral plexus. 



