258 
PROFESSOR D. WATERSTON ON 
The principal developmental changes have been completed at the 30-mm. stage, 
though complete separation of the atria does not occur until birth. 
The embryos examined in section include the following specimens : — 
1. Embryo 2Wl, 3 mm. in length (maximum). 
2. 
„ Si, 
6 
3. 
„ Bl, 
-8 
4. 
„ Ml,. 
9 
5. 
S4, 
12-5 
6. 
„ B2, 
13 
7. 
,, 
16 
8. 
„ S3, 
20 
9. 
„ B3, 
22 
10. 
,, 
30 
11. 
„ El, 
28’5 
All of these embryos showed good histological condition, and most of them 
almost perfect histological detail. 
I am indebted to Prof. Symington for the embryos Bl, B2, and B3, and to 
Prof. Peter Thompson and to Prof. Bryce respectively for the opportunity of 
examining and reconstructing from their sections the heart in the embryos 7 and 
10 of the list. 
In addition, I have examined series of sections of embryos of the cat, guinea-pig, 
and mouse, and I am greatly indebted to my colleague, Prof. W. C. MTntosh, for 
the opportunity of examining specimens of the adult heart in the shark, dogfish, 
boa, and crocodile, examination of which gives considerable help in the correct 
interpretation of the conditions found in different stages of the development of 
the mammalian heart. 
Prof. Peter Thompson also kindly allowed me to examine several of his recon- 
structions of developing hearts, and particularly his model of the 2 '5 -mm. embryo of 
B. Meyer’s collection, which is a most valuable specimen. 
The earliest stages of the development of the human heart are as yet unknown, 
as the material necessary for investigating them does not exist in any collection ; but 
it is presumed that the heart follows the lines of development exhibited' in the earliest 
stages of other mammals, in whom the primitive heart is formed by the fusion of two 
parallel longitudinal vessels, which unite to form a single tube, receiving at its caudal 
extremity the afferent veins, and terminating orally in a single vessel, the truncus 
arteriosus, which divides almost at once into the two ventral aortse. 
The heart tube becomes bent upon itself and twisted into the shape of an S. 
Alternate dilatations and constrictions make their appearance, so that it comes to 
show a series of dilatations, viz. the sinus venosus, the atrium, the ventricle, and the 
“ bulbus cordis” in succession from the caudal to the oral end. The atrium and 
