238 HUMAN PHYSIOLOGY. 



cell, from which the new being develops through successive division, multi- 

 plication and differentiation of cells. 



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

 continues to divide and pass slowly to the uterus (8-10 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 sur- 

 face. During the passage down the oviduct the zona pellucida 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 capsu- 

 laris or reflexa, that beneath the ovum the decidua basilaris or placentalis, 

 while the remainder constitutes the decidua parietalis or vera. As develop- 

 ment proceeds the decidua basilaris becomes greater and ultimately develops 

 into the placenta, the organ of nutrition and respiration. 



Segmentation of the Ovum. Immediately after fertilization the ovum 

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

 ular mass of cells 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 entoderm; and the 

 mesodermal or celomic cavity limited by the extra embryonic mesoderm. 

 Meanwhile cells in various parts of the thickened trophoderm have dis- 

 appeared 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 constitute the embryonic shield or disk. 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 temporary struc- 

 ture that is soon overshadowed by changes in the area just in front of it. 

 Here is formed a median longitudinal, grooved ridge of ectoderm that devel- 

 ops 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 

 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 



