As an embryo grows from a zygote through its various forms...
Genes in each cell turn on and off, changing the identity and fate of each cell.
If the timing of these changes is modified, even slightly, profound effects on the finished product can arise.
In short, if ontogenies diverge, then adult morphologies may also diverge.
Heterochrony (from the Greek hetero meaning "other" and chronos meaning "time") describes a change in the timing of ontogenetic events between two taxa. These can be the result of relatively small genetic changes that may not even be alterations in DNA sequence, but in the timing of particular genes being expressed during development.
A heterochronic change can result from a mutation that causes the rate of one cell line of the body to develop at a rate different from that of other cell lines in the body. This can result in allometric growth (from the Greek allo meaning "different" and metr meaning "measure" (and also, interestingly "womb")).
In a species that exhibits allometric growth, different cell lines/body parts grow at different rates (relative to an ancestral, isometrically growing form) during development from juvenile to adult.
Hold that thought.
Here's an analogy that might help you visualize this.
The difference between these two processes is subtle, but important to note when comparing a taxon's development to that of a closely related taxon, or to that of a hypothetical ancestor.
In either case, the body is still in its juvenile form when the animal becomes sexually mature, and body development then STOPS.
The resulting condition is known as paedomorphy (from the Greek paed, meaning "juvenile" and morph, meaning "form"): a reproductive adult that has a more juvenile form of the ancestral species.
Examples of paedomorphic organisms:
1. The Common Mudpuppy (Necturus maculosus) is a salamander that retains its juvenile gills as an adult
Most salamander species have aquatic larvae that lose their external gills when they reach adulthood: Juvenile Tiger Salamander (Ambystoma mabeei):
The mudpuppy is paedomorphic with respect to other salamander species: It retains its external gills as a reproductive adult due to either neoteny or progenesis:
2. Many domestic dog breeds (Canis lupus familiaris), derived from wolves (Canis lupus), exhibit paedomorphy with respect to adult wolves.
3. Homo sapiens, whose prolonged brain development period and relatively flat face reflect a prolonged juvenile period, relative to that of our closest relatives, the chimpanzees (Pan paniscus and P. troglodytes)
Although some humans tend to look more like our ancestors than others do.