Frontal section through the head of newborn - region of molars
The big empty space is where the brain would be, if it were left in the head, just to get a general orientation. The blue circle-shaped regions shown down near the tongue cross-section are odontoblasts (tooth germs). The deciduous (baby) teeth all begin their development early in gestation. By 20 weeks into pregnancy, the initial calcification has established the tooth germs throughout the mouth.
Though the crowns of the teeth (harder tissues - dentin and enamel) are not deposited until roughly 5-6 months old in the case of the first molars, you can clearly see the development of the inner tissues of the teeth going on in this cross-section.
Atlas and Textbook of Dentistry Including Diseases of the Mouth. Gustav Preiswerk, 1906.