This article has summarized parts of certain publica­tions about changes of hematopoietic elements, specially erythroid, during embryonic and fetal to the third postnatal life. The main purpose of this article is to familiarize medical students with the physiological characteristics of the blood system, chiefly in the first trimenon of postnatal life. 
It is also aimed to prevent many incorrect conclusions which could lead to remarkable diagnostic errors with unnecessary consequences of harmful treatments.