2 EMBRYOLOGY 



Students of other biological sciences and practices should realize that 

 great advances in many fields have depended upon embryological studies. 

 The cultivation of the virus in the tissues of chick embryos led to discoveries 

 concerning the inactivation of the virus under certain conditions. Yellow 

 fever virus attacks the nervous system, but if we culture the virus in non- 

 nervous tissues the virus changes to a form which is no longer virulent 

 toward nervous tissues. Animal breeding depends on fertilization studies. 

 Fish culture and wild-life conservation are concerned with the study of 

 breeding habits and the physiology of fertilization. Recently the technique 

 of tissue culture, which arose as an embryological method, has been used for 

 screening new compounds and new drugs with respect to their action on cell 

 division and growth. Dividing egg cells are used to study the effects of 

 radiation, and some eggs are used as single cells for studies of permeability, 

 viscosity, cell metabolism, and the behavior of chromosomes. The use of 

 sperm for the study of the chemical characteristics of the nucleoproteins goes 

 back to the nineteenth century. Spermatozoa are a very good source of an 

 important enzyme, hyaluronidase, which breaks down a jellylike substance, 

 mucin. The breakdown of mucin is one step in the process of fertilization. 



The general nature of development 



Introductory courses in zoology teach the structure of the organism, and 

 they show something of the complexity of structure. The nervous system, for 

 example, appears tremendously complex, with large numbers of cells and 

 many connections between the cells, forming an integrated system. Even ele- 

 mentary courses show the complexity of the chemical reactions that take place 

 in the cell during respiration and the synthesis of protoplasm. We are now 

 faced with the problem of tracing the derivation of this structural and bio- 

 chemical complexity from a single cell. It is just as well at the beginning not 

 to underestimate the magnitude of the problem. For example, the human 

 egg is. about 0.2 mm. in diameter. When we consider that this cell multiplies 

 itself into a newborn child weighing from 5 to 10 pounds, we get some idea 

 of the increase in size which must take place. If we calculate the weight of 

 the egg from its volume, the egg should weigh three one-millionths of a 

 gram. If we take the weight of the newborn child as 3000 grams, this means 

 an increase in weight of about one billion times. 



Or we can look at it another way. We begin with a single cell and end up 



