In experiments of disaggregation and re-aggregation of embryonic cells, there sometimes arise complexes in which differently differentiated tissues are present in chaotic arrangement, having no relation, or only a slight relation, to the arrangement of the same tissues in a normal individual. We have referred to such complexes as teratomas (Fig. 454). The term "teratoma," however, was originally coined to [...]
In this article, the changing pattern of processes of differentiation will be shown in examples taken from the development of living organisms. Hemoglobins: How specific genes change their activity during development is illustrated by the composition of hemoglobins in man. The hemoglobin in adult humans is in the form of composite molecules consisting of four units (polypeptides), two of which [...]
When development starts the genes are in a repressed stage. During cleavage the genes are inactive (with the exception of histone genes), and whatever hereditary traits can be observed are due to gene action during the growth and maturation of the oocyte. From gastrulation onward, some genes become activated, and the action of individual genes becomes evident. By observing normal [...]