egg tooth

Japanese: egg tooth（英語表記）eggtooth

…In echidnas, before hatching, a single unpaired tooth appears in the center of the front end of the upper jaw. This is called an egg tooth because it is used to break the shell, and it falls off after hatching. After that, it has no teeth at all and feeds on ants and termites. …

From [Hatching]

...Artificial incubation is also used in poultry and other animals. Examples of mechanical force used to break the egg membrane during hatching include the egg tooth found in the embryos of monotremes, birds, reptiles, and insects (crickets, dragonflies, fleas, etc.). This is a hard protrusion that forms on the embryo's snout or on the membrane that covers the embryo, and is used to break the eggshell.

*Some of the terminology that mentions "egg tooth" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…ハリモグラでは，孵化（ふか）する前に上あごの前端正中部に無対の1本の歯が生ずる。これは殻を破るのに用いられるので〈卵歯egg tooth〉と呼ばれ，孵化後に脱落する。その後はまったく歯をもたず，アリやシロアリを常食とする。…