Susanna Phelps G-age 4:17 



Dermatomes. — This specimen by itself does not throw much light on 

 the question as to the fate of the outer wall of the myotome, i. e., whether 

 it is in reality a dermatome or not. 



Vascular System. 



Heart. — In Figs. 1, 3, and 5 the position of the heart is seen, and in 

 Fig 12, a ventral view showing a compact, somewhat square form, remind- 

 ing one of the His' model from a 10 mm. (1 week) specimen rather than 

 those from younger specimens. The greater length of the right auricular 

 portion is similar to the His model of a 5 weeks' embryo. A very young 

 embryo, modeled and presented by J. L. Bremer " at the American Asso- 

 ciation of Anatomists, in December, 1904, shows an exaggeratedly long 

 right auricle. In external form, the heart of this specimen resembles 

 older, rather than younger stages, but the internal relations accord with 

 the descriptions given for the 3 weeks' stage. That is, the tubular heart 

 is already dividing into right and left (Figs. 2, 6) ; the entrance of the 

 sinus venosus is decidedly to the right (Fig. 3, left of Fig.), the con- 

 nection of the auricle with the ventricle is through a narrowed tube, the 

 auriculo-ventricular canal (Fig. 10), entering the ventricle at the left 

 (Fig. 6), while the bulbus arteriosus makes its exit at the right (Figs. 

 13, 6), turns sharply cephalad and extends along the ventral aspect of 

 ihe heart. 



The walls of the heart contain only undifferentiated muscular tissue. 

 In parts the endothelial tube is closely applied to the walls; in parts, 

 notably along the path of the auriculo-ventricular canal (Fig. 10), the 

 ventral end of the right ventricle (Fig. 6) and of the entire bulbus (Figs. 

 10, 2), the endothelial tube is connected with the outer wall by only a 

 •delicate mesh-work of tissue. 



Arteries. — The endothelial tube of the bulbus plunges into the floor of 

 the pharynx, expands into a wide sinus, giving otf on each side near the 

 middle line, a small branch which divides into the 1st and 3d gill arches 

 and a mesh-work of capillaries supplying the mandible. The 1st and 3d 

 arches join with the dorsal aorta by a very slender capillarv connection. 

 'The 3d and 4th gill arches (Fig. 11) are given off from the side of the 

 sinus and unite dorsally to form the main portion of the dorsal aorta. 

 The 5th gill arch is given off caudally near the middle line, divides into 

 •capillar}' branches, supplying the larynx and trachea (5th and 6th 

 arches), reaching the dorsal aorta by very small branches. Each dorsal 

 .aorta sends forward a branch which can be traced in the roof of the 



'2 Bremer, J L., Amer. Jour. Anat., IV, No. 2, p. VIII. 



