328 F. L. LANDACRE 



and extends somewhat anterior and posterior to the auditory 

 region, where it becomes merged completely with the dorso-lateral 

 mass to form the lateral mass. Posterior to the auditory vesicle, 

 in the region of the IXth nerve, it presents the appearance shown 

 in fig. 5. This ventrally differentiated mass shown in figs. 4 

 {Pre. PL), 5 (Post. PL) and 6 (Au. Ves.) develops later into the 

 auditory vesicle and the pre- and postauditory placodes (dorso- 

 lateral placodes). 



The fate of the lateral mass, as a whole, varies in different regions 

 of the head. In the regions between the optic stalk and the Gas- 

 serian ganglion it becomes converted entirely into mesectoderm. 

 Fig. 7 is taken from an embryo slightly older (Stage III) than that 

 from which figs. 2 and 3 were taken, and is identical in position 

 with fig. 2, with which it should be compared. The lateral mass is 

 here free from the cord on its mesial border nearly to the dorsal 

 surface of the cord; while on its lateral border it is free from the 

 epidermis up to about the same level. The ventral two-thirds 

 of the mass is converted into a rather loose mass of mesectoderm 

 in which the cell boundaries are indefinite and in which there are 

 numerous intercellular spaces. 



The later history of this mass shows that it is converted com- 

 pletely into a very loose mass of mesectoderm with large intercellu- 

 lar spaces and with faint cell boundaries, but with well defined 

 nuclei. I have detected during this change of the lateral mass into 

 mesectoderm no mitotic figures in any of my sections. Posterior 

 to the region in which the Gasserian ganglion forms and between 

 that ganglion and the lateralis Vllth the lateral mass is converted 

 chiefly into mesectoderm, except its ventral border which repre- 

 sents the forward extension of the primordium of the auditory 

 vesicle, or the preauditory placode. Fig. 8 from the same embryo 

 is taken through the region in which the Gasserian ganglion forms. 

 The lateral mass is here detached from the cord mesially, except 

 at its dorsal border, but it is attached to the epidermis throughout 

 its whole length. The dorsal and particularly the dorso-mesial 

 portion of the solid lateral mass is beginning to be converted into 

 a looser cell mass. Fig. 9 is taken just anterior to the auditory 

 vesicle in the position in which the lateralis Vllth ganglion will 



