322 J. A. BADERTSCHER 



terior portion of the entodermal thymus do not come in contact 

 with each other in this developmental stage. 



Embryo of 17.5 mm. {figures 3 and I^. During the interval 

 between this and the previous stage shiftings in the pharyngeal 

 region have taken place that have changed the relation of some 

 of the parts to each other. The ductus cervicalis {D.c), now a 

 solid cord of cells, is still connected with the ectoderm. The 

 ductus branchialis on the left side, which was not modelled, has 

 lost its connection with the outer end of the vesicula cervicalis. 

 No traces of it in this region can be seen. It is, however, still 

 connected with the second branchial pouch from which it extends 

 for a short distance toward the point of its former attachment. ^ 

 On the right side it is still a continuous solid cord of cells ex- 

 tending from the second pouch to the vesicula cervicalis. In a 

 21 mm. pig embryo Zotterman ('11) had demonstrated the ductus 

 branchialis as a continuous cord of cells while the ductus cervi- 

 calis as being broken. In the embryos which I examined the 

 ductus branchialis was always the first to become discontinuous. 



The vesicula cervicalis {V.c.) no longer lies perpendicular to 

 the ectoderm. The vesicula cervicalis medialis {V.c.vi.y extends 

 from its point of attachment to the ganglion nodosum in an 

 antero-lateral direction to the hypoglossal nerve (N.XII) around 

 which it forms an acute angle. Fr©m the nerve the vesicula 

 cervicalis lateralis {v.c.l.) extends for a short distance in a caudo- 

 lateral direction. This is the same general direction taken by 

 the ductus cervicalis which is connected to the vesicula cervi- 

 calis laterahs and the ectoderm. The vesicula cervicalis medialis 

 is tightly wedged in between the vagus on its dorsal side and 

 the parathyroid gland and a small portion of the thymus on its 



2 A reconstruction of this remnant was deemed unnecessary since it takes no 

 part in the formation of the thymus and would have needlessly increased the 

 size of the model. 



^ From the reconstruction as represented in figures 3 and 4 it will be seen that 

 the vesicula cervicalis now loops over the hypoglossal nerve. For the sake of 

 simplicity as well as for clearness, that portion of the vesicula cervicalis lying 

 between the nerve and the pharynx will be termed the 'vesicula cervicalis medialis' 

 while the part lying between the nerve and the surface ectoderm will be termed 

 the 'vesicula cervicalis lateralis.' 



1 



