620 



Jacob Parsons Schaeffer. 



Epithelial plugs occlud- 

 ing "Stares 



EXPLANATION_OF FIGURES 



Figs. 3, 4, 5, (33.2) Photomicrographs of frontal section.s through the head of 

 an embryo aged approximately 35 days (Human embryo, No. 26, Cornell Univer- 

 sity series, slides 19 and 20). The sections are in the region of the primitive nasal 

 fossae. Section fig. 3 is the most ventral and section fig. 5 the most dorsal of the 

 three. 



Fig. 3. Note that in the region of the coalescence of the medial nasal and the 

 ma.xillary processes there is no intervening ectoderm. Fusion is complete and 

 absolute, in that the mesenchymal tissue of the maxillary and medial nasal 

 ]>rocesses is continuous. 



Fig. 4. Note the ectodermal strands between the medial nasal and the maxil- 

 lary processes; the mesenchymal tissue of these processes is not continuous. 

 Compare this condition with fig. 3. 



Fig. 5. On both sides the two layers of epithelium, oral and nasal, have become 

 attenuated and thinned out to represent single layers of cells. These are the 

 bucconasal membranes which ultimately rupture to establish the primitive 

 choanae. 



Fusion, = coalescence between the maxillarv and the medial nasal proce.sses; 

 Med. Has. proc, = medial nasal process; Max. proc. = maxillary process; Menib. 

 hue, — membrana bucconasalis. 



Fig. 6 (X 18) Photomicrograph of a frontal section through the ventral por- 

 tion of the nose of an embryo aged approximately 49 days (Human embryo. No. 

 28, Cornell University series, slide 40). 



Shows the epithelial plugs occluding the nares. This is a rather common con- 

 dition during the latter part of the second month and throughout the third month. 

 It also occurs after this period, as I have seen it in a 120 day-eml>ryo. 



