900 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



c. Types of placental relationships in the eutherian mammals 



1 ) Epitheliochorial type 



2) Endotheiiochorial variety 



3) Endotheiiochorial plus syndesmochorial placenta 



4) Hemochorial placenta 



5) Hemoendothelial placenta 



3. Implantation of the human embryo 



a. Preparation for implantation 



b. Implantation 



c. Formation of the placenta 



4. Implantation in the rhesus monkey, Macaca mulatta 



5. Implantation of the pig embryo 



6. Fate of the embryonic membranes 



a. Yolk sac 



b. Amnion and allantois 



D. Functions of the placenta 



E. Tests for pregnancy 



1. Aschheim-Zondek test 



2. Friedman modification of the Aschheim-Zondek test 



3. Toad test 



4. Frog test 



F. The developing circulatory system in relation to nutrition, etc. 



G. Post-hatching and post-partum care of the young 



A. Introduction 



1. Care in Relation to the Number of Young Produced 



In this chapter, we shall consider the methods by which developing embryos 

 of different vertebrate species are cared for and nourished during develop- 

 ment. The amount of care given to the developing egg varies greatly. How- 

 ever, one primary rule appears to govern the reproductive habits of the species, 

 namely, the species must survive. This survival is accomplished by two prin- 

 cipal methods: 



( 1 ) by the production of enormous numbers of developing young, given 

 no protective care, with the result that few survive to the adult or 

 reproductive stage, and 



(2) by the formation of fewer developing individuals with greater amounts 

 of protective care. 



Generally speaking, the fewer the individual embryos produced, the greater 

 the care. 



Examples of the method of species survival without parental care are evi- 

 dent in the codfish, Gadus, which spawns about 8 to 10 millions of eggs during 

 a particular breeding period or in the ling, Molva, which discharges from 

 14 to 60 millions of eggs at one time. In these instances, the species survive 

 by the sheer number of developing young produced. On the other hand, the 

 shark, bird, and mammal substitute an extreme care of the developing egg, 



