CHAPTER 31 



The Development of Mammals 



We shall conclude our consideration of the organ systems of verte- 

 brates by briefly examining the embryonic development of the organs. 

 The general features of vertebrate development were discussed in 

 Chapter 6 and should be reviewed at this time. We shall focus our atten- 

 tion on the early stages in the development of mammals, which differ 

 in some respects from those of other vertebrates, and on the establish- 

 ment of the organ systems. 



260. Early Stages of Mammalian Development 



Monotreme embryos derive their nutrients in reptilian fashion 

 from the large accumulation of yolk stored in the cleidoic egg, but 

 other mammalian embryos develop within the uterus and derive their 

 nutrients from the mother through the placenta. These mammals do 

 not provide their eggs with much yolk. The eggs are isolecithal and so 

 small that they can barely be seen with the unaided eye. Indeed they 

 are so small that the early stages of mammalian development remained 

 a mystery long after the early development of other vertebrates had 

 been described. William Harvey, famed for his discovery of the circula- 

 tion of the blood, searched the uteri of deer in vain for early embryos, 

 and finally concluded that the embryo might somehow be secreted by 

 the uterus when seminal fluid was introduced. In 1672, de Graaf dis- 

 covered early cleavage stages (he called them eggs) in the Fallopian 

 tube of a rabbit, and concluded, correctly, that the eggs came from the 

 ovary. The first mammalian egg to be seen, a dog's egg, was observed 

 by von Baer in 1827. Human eggs free within the Fallopian tube and 

 early developmental stages have been described only in recent years. 



Cleavage might be expected to be a very regular process in mam- 

 malian eggs as it is in other isolecithal eggs. The mammalian egg does 

 cleave completely, and the first two or three cleavages in primates are 

 regular and produce blastomeres of nearly equal size (Fig. 31.1). Sub- 

 sequently, certain blastomeres divide faster than others, and cleavage 

 becomes somewhat irregular. This may be a reflection of the irregular 

 cleavage characteristic of the reptilian telolecithal egg, which was, of 

 course, the type of egg present in mammalian ancestors. 



a' solid ball of cells, the morula, is produced, and as the cells 

 continue to divide, they arrange themselves about a central cavity. 



637 



