152 EDWARD MCCRADY, JR. 



The six rhombic grooves are quite conspicuous in the floor of 

 the hind-brain. The lower eyelid fold is recognizable. Epi- 

 trichial cells are making their first appearance. 



During stage 32 mesoderm invades the interval between the 

 pericardium and the ectoderm so that the ventral and lateral 

 thoracic walls become opaque. Within this mesenchyme the 

 anlagen of sternal bars, ribs, and coracoid become differ- 

 entiated. 



Heuser studied two of The Wistar Institute specimens of 

 this stage (16173 and 16174) and the following notes are taken 

 from his abstract ('19): "The ganglion nodosum is fused 

 with the cervical sinus, and there arises from the ganglion a 

 cord of cells which, with a strand from the thymus, makes 

 another connection with the sinus. The hypoglossus is em- 

 bedded in the nodosum, but does not touch the thymus. The 

 superior laryngeal nerve is distinct: it extends from the 

 mesial border of the cord above referred to and runs between 

 the thymus and the parathyroid." All of these points are 

 illustrated in figure 47, G, which is taken from his wax model. 



In the heart there is a large septum spurium and no 

 septum secundum. The foramen ovale has already been de- 

 scribed. The interventricular septum is forming but is not 

 large (fig. 47, B). The pulmonary trunk has twisted about 

 the aorta until it leaves the conns on the left instead of the 

 posterior side. 



The second half of the twelfth day 



Stage 33. External distinctive features. The veins of the 

 Jiver and the postcava. The septum primum. The jugular 

 lymph sacs. The thoracic supracardinal (azygos) veins. 

 The paravertebral lymphatics and the thoracic duct. The 

 migration of the gill pouch derivatives. The allantois at its 

 height. Miscellaneous details. 



External distinctive features. Stage 33 is distinguished 

 from stage 32 by the following details : upper and lower eyelid 

 folds are quite distinct ; there are real digits, not just digital 

 buds, on the forefoot; the beginning of the pontine flexure 



