108 TEXT-BOOK OP EMBRYOLOGY. 



cavity. These cells comprise the primitive entoderm. Meanwhile the cen- 

 tral cells of the inner cell mass undergo vacuolization, leaving now only the 

 enveloping layer and a single layer of cells applied to the entoderm. This 

 single layer is the embryonic ectoderm and the newly formed space the amniotic 

 cavity (Fig. 59, c). The entire structure is known as the blastodermic vesicle 

 or blastocyst; the interior contains two cavities separated from each other by 

 a plate or disk composed of ectoderm and entoderm and called the embryonic 

 disk. At this point it must suffice to say, without entering into details, that 

 the mesoderm appears as a third layer between ectoderm and entoderm in 

 the embryonic disk and between entoderm and enveloping layer. The meso- 

 derm increases rapidly and soon forms an extensive but loosely arranged tissue 

 between the entoderm and the enveloping layer in the wall of the vesicle. 

 The enveloping layer becomes known as the trophoderm because it comes in 

 direct contact in the mammal with the uterine mucosa and through it must 

 pass all the nutritive materials from the uterus to the interior of the vesicle. 



Up to the time and stage when the mesoderm becomes a loosely arranged 

 tissue filling much of the interior of the blastodermic vesicle, nothing is 

 known of the development of the human ovum. What is probably the 

 youngest human embryo, described by Bryce and Teacher, is shown in sec- 

 tion in Fig. 73. The trophoderm is. the outer layer of the vesicle and has 

 sent out numerous irregular projections into the uterine mucosa in which the 

 vesicle is already embedded. The interior of the vesicle is occupied for the 

 most part by the loose mesoderm. Embedded in the mesoderm are two cav- 

 ities, the smaller being the yolk cavity lined by entoderm and the larger the 

 amniotic cavity lined by ectoderm ; the cavities are separated from each other 

 by the embryonic disk. This embryo was reckoned to be 13 or 14 days old. 



A slightly older human embryo has been described by Peters (Fig. 74). 

 It is now reckoned to be about 15 days old, although Peters regarded it at 

 the time as being much younger. The trophoderm exhibits about the same 

 characters as in the Bryce-Teacher embryo. The mesoderm shows a great 

 cleft or space within it; a rather thin layer is applied to the trophoderm and 

 also surrounds the yolk and amniotic cavities and forms the middle layer of 

 the disk between the two cavities. The space within the mesoderm is the 

 exoccelom or extraembryonic body cavity. The layer applied to the tropho- 

 derm is the somatic or parietal mesoderm which with the trophoderm itself 

 comprises the chorion. The wall of the yolk sac is composed of entoderm 

 and visceral or splanchnic mesoderm. The amniotic cavity is surrounded 

 by ectoderm and parietal mesoderm. The embryonic disk is attached to 

 the chorion at one side by a strand of mesoderm known as the belly stalk. 

 The chief difference between this and the Bryce-Teacher embryo is the great 

 cleft in the mesoderm. 



