The Limit Between Eetoderm and Entoderm. 61 
growing ectoderm. ‘The fact that this ectoderm forms dental ridges 
does not wholly exclude the supposition that some of the cells might 
give rise to taste buds. Therefore, an account of the mode of forma- 
tion of taste buds in the front part of the mouth will be sufficient to 
determine whether any of the taste buds are derived from ectoderm. 
Fig. 19 shows a taste bud forming in the floor of the mouth a 
short distance behind the mandibular teeth at the time when the 
mouth cleft is forming. The ectoderm is given a darker shade than 
Fie. 19. A. punctatum, sagittal section near the median plane shortly before 
the formation of the mouth opening. The section includes a part of the dental 
ridge in the mandible and the tip of the hyoid arch. Between the two is seen 
a taste bud forming in the entoderm. The asterisk marks the caudal limit of 
the ectoderm of the vomerine tooth-germs. Iron hematoxylin, fuchsin. 
the entoderm and all the yolk granules are drawn. For a short dis- 
tance in the middle of the figure the nuclei are drawn. A small iso- 
lated opening indicates the forming mouth cavity. Below it appear a 
group of elongated nuclei placed vertical to the floor of the mouth. 
This grouping of nuclei is the first indication of a taste bud. The 
outlines of the cells are difficult to see on account of the great number 
of yolk granules. This taste bud is forming in the entoderm between 
the tongue and the teeth in the floor of the mouth. It is surrounded 
on all sides by entoderm crowded with yolk granules and these gran- 
