914 



EMBRYOLOGY. 



the same origin ; the cutaneous muscles, wliich are developed at the expense 

 of the cutaneous laminse of the middle layer of the blastoderm ; and the 

 muscles of the limbs, whose development is not yet perfectly known. 



It was at one time believed that the muscular fibres were formed by the 

 junction, end to end, of several elongated cells ; but it is now known that 

 they are constituted by a single cell which lengthens, and whose nuclei 

 multiply and lie at the surface, while its contents are transformed into a 

 substance which oifers the characteristics of contractile tissue. The sar- 



colemma is formed after the fibre, 

 Fig. 440. by a modification of the con- 



nective tissue surrounding it. 



DEVELOPMENT OF THE OIRCULATOKT 

 APPARATUS. 



During the first days which 

 follow the appearance of the em- 

 bryo in the substance of the blas- 

 todermic layer, there is no trace 

 of vessels in the area germinativa. 

 It is not long, however, before the 

 heart and some blood-vessels are 

 seen in the middle layer, the 

 vessels extending to the surface 

 of the umbilical vesicle, which 

 gradually shows itself. While 

 the contents of the vitelline vesicle 

 are being absorbed by the embryo, 

 the heart is being completed, the 

 vessels are developing, the allan- 

 tois is formed, and the placental 

 circulation, which continues until 

 birth, is established. From this 

 time the circulatory apparatus has 

 acquired its definitive disposition. 



1. Appearance op the Heart. 



diagram of the fokmation of the ven.e p 



omphalo-mesenterioie and timbilicales. circulation in the umbilical 



1, At the time of the first appearance of the urn- VesICLE.— The pleuro-peritoneal 



bilicalcs, and the commencement of the om- cavity of the embryo presents, 



phalo-mesenterica; ; 2, At the time of the first anteriorly, a diverticulum, the car- 



appearance of the branches to and from the diac cawfa, in the interior of which 

 liver, and the dimmution of the oraphalo- ,, i . • i i j rrn • 

 mesenteric vessels ; 3, 4, At the period of com- t^S ^l^art IS developed. IhlS 

 plete fcetal circulation in 1, Omphalo-mesenteric organ appears at first as a Small 

 trunk; in 2, 3, Remains of it; in 4, Vein of the masS of cells, the innermost of 



yott sac alone : om', Right, and om" Left vena ^jji^ji become separated from each 



omphalo-mesentericas ; u. Trunk of the um- .i . , -j. j 



bilical vein; < Right, and u", Left vena un». "tjier to Create a cavity and con- 



bilicalis ; do, Ductus Cuvieri ; j, Jugularis ; c, stltute the blood-globules. As SOOn 



Cardinalis ; I, Liver ; ha, hepatica advehentes ; aS it shows itself, the heart COn- 



hr, Hepatica; revehentes ; m, Mesenterica; ; da, tracts and dilates alternately, the 



fuctus venosus Arantii ; ci. Cava inferior ; », .„ ,„ 4. i, ■ 1 ±\. i, 



^-ena port^ ; I, Lienalis; m, Mesenterica 'su- movements being very sloW, though 



pcnor. they gradually become quicker. 



Towards the twelfth day, the 

 central organ of the circulation has the form of a contractile cylindrical tube. 



