30 GENERAL EMBRYOLOGY. 
parietal layers of the pleural and peritoneal membranes, and with the development 
of the connective tissues, fascize, and vessels of the ventro-lateral walls of the body ; 
and in mammals, apparently, it also gives origin to the ventro-lateral body muscles 
and the muscles of the limbs. The splanchnic portions of the lateral plates form 
the fascive, the connective tissues, the smooth muscles of the walls of the alimentary 
canal, the heart and great blood vessels, the visceral layers of the pleural and 
peritoneal membranes, the spleen, and the germinal epithelium, which becomes 
transformed into the mother cells of the ova and spermatozoa. 
In the cephalic region, in higher vertebrates, lateral plates are not recognisable, 
except so far as they may be represented by the walls of the pericardium; but in 
some lower vertebrates lateral plates can be distinguished, corresponding in 
number with the cephalic somites, and it has been asserted that the muscles of the 
Hind brain 
y Auditory ganglion 
“  _~ Rudiment of otic vesicle 
Paraxial se 
Hyo-iandibular 
eleft 
\ 
f > 
ie ee 
First cephalic 
aortic arch 
Fic. 24.—TRANSVERSE SECTION OF A Rat EMBRYO. 
Showing the relation of the paraxial mesoderm of the head to the lateral plates, the commencement of the 
formation of the otic vesicles and hyomandibular clefts, and the relation of the primitive heart to the 
pericardium and fore-gut. 
EC. Ectoderm. SoM. Somatic mesoderm. SpM. Splanchnic mesoderm. 
face and the muscles of mastication are developed from the lateral plates associated 
with the second and third cephalic somites. The subject, however, is one which is 
still obscure, and requires further investigation before any very positive conclusion 
can be arrived at. 
The Intermediate Cell Mass.—As already mentioned, the lateral plates and 
the protovertebral somites are connected by the intermediate cell-masses, which are 
intimately associated with the development of the ducts and tubules of the genital 
and urinary organs in man and other mammals. On each side the mass soon 
separates from the protovertebral somites, and is transformed by rapid proliferation 
of its cells into an elongated body, the Wolffian body or primitive kidney, which 
projects downwards into the dorsal angle of the body cavity. In early stages it 
extends from the fifth somite of the body backwards to its posterior end, but is 
most clearly differentiated in the middle portion. The Wolffian duct and tubules 
and the Miillerian duct are developed in connexion with it; after the second month 
of intrauterine life it degenerates, and is replaced by the permanent kidney, which 
is formed dorsal to its posterior extremity. 
