46 GENERAL EMBRYOLOGY. 
and cutaneous lamelle in the thoracic and pelvic regions, and as the muscle-plates 
pass downwards in the somatopleure towards the ventral aspect of the body, these 
buds grow outwards into the limb-rudiments and develop into the muscles of the 
limbs. Presumably this is the more primitive arrangement, and that met with in 
man and other mammals is secondary, and it is stated that although no distinct 
buds from the muscle-plates pass into the limbs of mammals nevertheless the 
limb-muscles are formed by cells, proliferated from the muscle-plates, which have 
migrated into the somatopleural mesoderm of the limbs. 
THE NUTRITION AND PROTECTION OF THE EMBRYO 
DURING ITS INTRAUTERINE EXISTENCE. 
The impregnated ovum during its passage down the Fallopian tube, and for a 
brief period also after it enters the uterus, lives either on the yolk granules (deuto- 
plasm) embedded in its own cytoplasm or upon material absorbed ‘from the fluids 
by which it is surrounded. The human ovum is very small, and consequently it is 
almost from the first dependent for its nutrition upon sources of supply outside 
itself. The urgent necessity for adequate arrangements whereby this may be 
effected leads to that early establishment of an intimate vascular connexion 
between the embryo and the mother which is so characteristic a feature in the 
development of the human ovum. At the end of the second week, after fertilisa- 
tion of the ovum, the embryo is separated by a slight constriction from the rest of 
the blastodermic vesicle, and already a primitive heart and rudimentary blood- 
vessels are distinguishable. 
The development of the vascular system, and the establishment of the foetal 
circulation, however, cannot well be understood until the formation and structural 
features of the group of closely associated extra-embryonic organs or appendages 
have been considered. 
This group includes the yolk-sac, the chorion, the amnion, the allantois, and 
the placenta. 
THE FQ@TAL MEMBRANES AND APPENDAGES. 
The Yolk-Sac or Umbilical Vesicle.—That portion of the blastodermic cavity 
and its wall which is not included in the body of the embryo to form the primitive 
alimentary canal constitutes the umbilical vesicle or yolk-sac. Its walls, like its 
cavity, are continuous with the corresponding parts of the intestine, and their 
structural features are identical, there being an inner layer of entodermal cells and 
an outer layer which is formed by the splanchnic layer of the mesoderm. 
In the human embryo the yolk-sac is a small flask-like body, suspended from 
the ventral wall of the alimentary canal by a hollow stalk, the vitello-intestinal 
duct, which passes through the umbilical orifice. It lies in the extra-embryonic 
continuation of the body-cavity (ccelom), and is filled with fluid. Possibly the 
contents of the yolk-sac are utilised in the nutrition of the embryo in its earliest 
stages, and the first rudiments of the blood-vascular system, viz. blood corpuscles 
and vessels, appear in its walls. In the human embryo, however, it is of little 
nutritional importance; it soon atrophies and almost entirely disappears, but leaves 
traces of its existence in the umbilical cord. 
The Amnion.—The amnion is a protective sac which surrounds the embryo. 
It is formed, after the development of the ccelom, from the amniotic area of the 
blastoderm, and its wall is continuous, at the margins of the umbilical orifice, with 
the body-wall of the embryo. Both walls consist of a layer of ectoderm and a 
layer of somatic mesoderm, but whilst in the body-wall the ectoderm is external 
and the mesoderm internal, the relative positions of the layers are reversed in the 
amnion, the mesoderm being external and the ectoderm internal. 
The cavity enclosed between the amnion and the embryo, the amniotic cavity, 
is filled with fluid, the amniotic fluid, in which the embryo floats. The amniotic 
cavity is quite shut off for some time from all the neighbouring spaces, but after 
