CHAPTER VII. 

 EARLY MAMMALIAN DEVELOPMENT. 



It is perhaps unfortunate for the student beginning the study of embry- 

 ology that no mammalian form can be taken for the early developmental 

 stages and regarded as wholly typical of the subclass. While there are 

 certain fundamental principles of development in common in all placental 

 mammals, there are also features which vary in different orders. In this 

 chapter no attempt will be made to set forth one line of development as 

 typical, nor will all the variations in the different orders be presented. We 

 shall attempt to present the fundamental principles as exemplified in certain 

 of the mammalian forms and to sketch briefly the early stages of human 

 ontogeny. 



The mammalian ovum represents the meiolecithal type in which there is 

 only a small quantity of yolk. Taking the human ovum as an example, 

 the cell is approximately two-tenths of a millimeter in diameter and is not 

 truly spherical but slightly ovoid in shape (Thomson). In section it presents 

 the appearance of the traditional typical cell (Fig. i). The cytoplasm is 

 coarsely granular owing to the presence of suspended globules of deutoplasm 

 or yolk. Most of the yolk globules are congregated near the center of the 

 cell, around the nucleus, while the peripheral cytoplasm is nearly destitute 

 of yolk. The nucleus is slightly eccentric and exhibits a distinct nuclear 

 membrane, a single plasmosome and rather scanty chromatin. 



In most mammals on which observations have been made maturation 

 or reduction of chromosomes begins in the ovary, the first polar body being 

 formed before the Graafian follicle ruptures. The second polar body is 

 formed after the ovum escapes from the follicle; in the mouse for example it 

 is extruded when the ovum reaches the oviduct and after the sperm has 

 entered the cytoplasm. According to recent observations made by Thomson 

 on an extensive series of human ovaries, both polar bodies are extruded by 

 the ovum prior to ovulation. The phenomena of ovulation are discussed 

 in the chapter on "The Germ Cells," p. 23, et seq. 



It is generally agreed that the normal site of fertilization of the mammalian 

 ovum is the upper (or outer) third of the oviduct or Fallopian tube. The 

 spermatozoa pass from the vagina through the uterus and into the oviducts 

 where they remain viable and capable of fertilizing for a considerable period 

 of time, perhaps several days or even weeks. In the oviduct the ovum is met 

 by numerous spermatozoa and one of the latter penetrates the egg cytoplasm. 



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