THE DEVELOPMENT OF THE HEART IN MAN. 
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ventricular ring into the interior of the cusp ; (3) a layer of rudimentary muscle 
tissue continuous with the atrial wall. 
The first and third layers merge into one another, and beyond that level the valve 
cusp consists of a single rather thick layer of muscle tissue, and it is covered on its 
atrial surface by an oval mass of endocardial cushion tissue. 
There are no chordae tendinese, but the muscle tissue of the flap is attached by a 
number of muscle strands to the trabecular tissue of the ventricle. The septal 
cusp shows a somewhat similar arrangement, its ventricular surface being formed of 
muscle tissue covered by a large mass of endocardial cushion tissue, but the auricular 
wall does not form any part of the substance of this valve cusp. 
The cavity of the right ventricle is continued upwards in a spiral fashion to 
become continuous with the pulmonary artery ; the trabecular arrangement of fibres 
being distinct on the ventral wall, while the dorsal wall is largely composed, not of 
muscle tissue, but of fibrous tissue derived from the septum separating it from the 
aortic stem. The muscle wall is continued to the base of the semilunar cusps of 
the pulmonary valve and ceases abruptly, the wall of the pulmonary artery and of the 
ascending aorta consisting quite definitely of fibrous tissue only. Proximal to the 
semilunar cusps for a distance of less than an inch there is the same narrowing of 
the lumen noticed in earlier specimens ; the stem of the pulmonary artery to its 
bifurcation now measures 45 to 50 mm. The wall of this part is undulating, showing 
two constrictions, as was seen in earlier specimens. 
The cavity of the left ventricle is very small ; medial and lateral cusps can be recog- 
nised at the atrio-ventricular orifice, and the aorta takes origin from a definite infundi- 
bulum corresponding in extent to the pars membranacea septi (text-figs. 15 and 16). 
The aortic valves lie again immediately above the upper margin of the mem- 
branous part. 
The wall of the right ventricle is in the model 30 mm. in thickness, while that of 
the left is 45 mm. 
Embryo 28‘6 mm. in Length (El). (Text-figs. 17 and 18.) 
The sections and the model of this singularly perfect specimen constitute a good 
picture of the heart at the close of the period of development specially studied, 
a stage when all the principal developmental changes of intrauterine life have 
been completed. 
In general shape the heart externally resembles that in the 30-mm. embryo. 
Internal Structure. — The atrial portion is partly divided into three portions — a 
central and two lateral, of which the central smaller chamber communicates with 
those on either side, while they in turn open into the corresponding ventricles. 
The right venous valve, as in the former specimen, forms a large vertical septum. 
In structure, especially at its basal attached margin, it differs from the atrial wall — 
the tissue is not striated, the cells are larger, and vacuoles are present. Along its 
