748 TEXT-BOOK OF PHYSIOLOGY 



The germ nucleus proceeds to the middle of the ovum where it is followed 

 by head and middle-piece of the spermium; the middle-piece forms a central 

 spindle while the germ nucleus and head of the spermium each resolves 

 itself into one-half the number of chromatic loops of a somatic cell. In this 

 condition the fertilized ovum represents a parent cell, that possesses the 

 physiologic activities and characters of both ancestral cells. From this 

 parent cell the offspring develops through successive division, multiplication 

 and differentiation of the resulting cells. The chromatic material of the 

 germ nucleus and head of the spermium represent the transmitters of in- 

 herited characters. 



The Fixation of the Ovum. The ovum, after fertilization in the 

 oviduct, continues to divide and pass slowly to the uterus (8 to 10 days) 

 where it is retained until the end of gestation. A menstrual mucosa having 

 developed the ovum lodges on a smooth thick area and gradually sinks 

 beneath the surface. During the passage down the oviduct the zona pellu- 

 cida has become attenuated and has been finally replaced by a thick layer 

 of ameboid and phagocytic cells called the trophoderm. Upon lodgment 

 of the ovum these cells destroy the underlying mucosa and produce a cavity 

 into which the ovum sinks. As the ovum increases in size the mucosa 

 gradually covers it; that portion of the mucosa toward the uterine cavity is 

 called the decidua capsularis (d. reflexa), that beneath the ovum the decidua 

 basilaris (placental d.), while the remainder constitutes the decidua parietalis 

 (d. vera}. As development proceeds the decidua basilaris becomes greater, 

 ultimately developing into the placenta. 



Segmentation of the Ovum. Immediately after fertilization the ovum 

 divides and redivides, within the diminishing zona pellucida, forming an 

 irregular mass called the morula. The peripheral cells form a layer, the 

 trophoderm, beneath the attenuated zona pellucida ultimately replacing 

 that structure. The remaining cells of the morula differentiate into three 

 masses ectodermal, entodermal and mesodermal; the central cells of these 

 masses liquefy and disappear forming thus the ectodermal or amniotic 

 cavity, limited by the ectoderm; the entodermal cavity, limited by the ento- 

 derm; and the mesodermal or celomic cavity, limited by the extra-embryonic 

 mesoderm. Meanwhile cells in various parts of the thickened trophoderm 

 have disappeared leaving this layer in the form of delicate trophodermal 

 villi, the future chorionic and placental villi. 



The Embryonic Shield. The floor of the amniotic cavity, consisting 

 of ectoderm and entoderm, constitutes the embryonic shield or disc. As 

 the shield increases in size a median longitudinal thickening is seen 

 occupying the caudal half of the area. This is the primitive streak, a tem- 

 porary structure that is soon overshadowed by changes in the areas just in 

 front of it. Here is formed a median, longitudinal, grooved ridge of ecto- 

 derm, that develops rapidly in length. This is the neural groove and folds. 

 The dorsal lips of the groove approach each other in the mid-line and fuse, 

 separating from the original ectoderm which closes over the ectodermal 

 tube. This ectodermal tube is the neural tube from which the nerve system 

 is developed. 



In the immediate vicinity of the head end of the primitive streak is seen 

 a darkened area, Hensen's node that represents the beginning invagination 

 of the ectoderm in the formation of the embryonic mesoderm and notochord 



