128 EDWARD MCCRADY, JR. 



ganglion cells or of differentiation of the three layers within 

 the spinal cord. In stage 30 all of the sensory ganglia for 

 the limb segments become organized out of the neural crest 

 and develop connections both with the cord and with the limb 

 bud ; and motor ganglion cells within the cord send out axones 

 to the limb bud. After noticing the striking similarity be- 

 tween the cranial, cervical, and thoracic parts of the body in 

 stages 29 and 30, to which attention has already been called, 

 one has only to compare section G of figure 39 with section H 

 in figure 42 to see at once that the principal developments in 

 stage 30 are in the posterior end of the body and especially 

 in the spinal cord. 



Bronchiolar buds. In chapter X the development of the 

 lungs was followed from the earliest, dorsolateral, paired 

 anlagen to the stage where they become ventral buds con- 

 nected to the pharynx by means of a single, ventral tube the 

 trachea. In stage 30 the process of bronchiolar budding be- 

 gins. This seems best described as monopodial. The original 

 bud on each side elongates to form a stem bronchus, and as 

 it grows caudad secondary bronchial buds arise as diverticula 

 from its side. In stage 30 one such secondary bud has arisen 

 on the left and two on the right. This stage corresponds to 

 that of a 10-mm. pig or a 7-mm. human embryo as far as the 

 lungs are concerned; but as the lungs in the opossum are 

 very precocious this comparison does not apply to the embryo 

 as a whole. For the less specialized organs the stage 30 

 opossum is better compared with a 5-mm. pig or a 104-day 

 (5 mm.) rabbit. 



This budding goes on principally in the endodermal por- 

 tion of the lung. The mesodermal blastema seems to receive 

 quite passively the general shape of the endodermal tree as 

 the latter expands within it; and the lobes are not well indi- 

 cated on the external surface of the lungs until a later stage. 



The pulmonary arteries. The origin of these arteries in 

 the opossum has not heretofore been described. Bremer's 

 remarks about them in his 1909 paper do not refer to the 

 early stages. Of more interest is his description ('12) of the 



