204 G. W. BARTELMEZ 
in this. Ingalls (20, p. 67) likewise identified it in his slightly 
older specimen (Carnegie Collection no. 1878), and with greater 
certainty, as he had several of our models of older stages for com- 
parison. Figure 1 shows the conditions in the first of our present 
series, a four-somite embryo (H279) with wide open neural folds. 
The dorsal part of the fold near the beginning of the second 
subdivision of the hindbrain (cf. p. 207) is enlarged; a swelling 
protrudes toward the ectoderm, ac. fac. gang.; and there is a 
Fig. 1 A photomicrograph of section 54 of the four somite embryo H27 
(Univ. of Chicago Coll.). xX 100. The plane of section is almost horizontal to 
the hindbrain. ac. fac. gang., anlage of the acousticofacial ganglion. The over- 
lying cap of ectodermal cells is more deeply stained than the ganglion. of. pl., 
otic plate; am., amnion; y. s., yolk sac. 
corresponding ventricular sulcus which does not show clearly 
in the photomicrograph. This is very like the condition described 
by Schulte and Tilney (’15) in the cat. The enlargement is 
termed acousticofacial ganglion in accordance with the customary 
mammalian usage (cf. p. 214). The ganglionic anlage is capped 
by a single layer of ectoderm cells which appears as if it 
had slipped over the top of the neural fold.. The adjacent ecto- 
derm is obviously thickened as the otic plate, ot. pl. The section, 
which is nearly horizontal through this region, shows almost the 
