Birds can be considered as warm blooded reptiles. At some stage in their evolution some of the mammals became viviparous, the embryo receiving an adequate supply of nutrition from the mother while it was retained in the uterus, and the yolk supply of the egg became superfluous. The yolk supply consequently became progressively reduced and eventually disappeared altogether. In this respect, however, there is a distinct gradation in the three mammalian subclasses. There is some uptake of nutrients from the uterus through the shell of the egg, prior to laying, but the eggs also have a large amount of yolk.
Cleavage is incomplete and the subsequent development follows essentially a reptilian pattern.
The yolk has already become superfluous. Its quantity is insignificant and is not used by the embryo but is ejected at the beginning of cleavage as the egg divides into the first two blastomeres. After birth the young remain in the pouch of the mother, nourished by milk, for a long period of time as they develop. With the disappearance of the yolk, the eutherian eggs have reverted to complete cleavage, but the subsequent development bears ample evidence of the former presence of yolk, and in many respects resembles the development found in eggs with a blastodisc type of incomplete cleavage, such as in the chicken. Examine demonstration slides of 56-96 hour chicken embryos.