cardiac jelly

https://doi.org/10.1351/goldbook.10490

Gelatinous substance, present between the endothelium and myocardium of the heart in early embryos, that develops into the connective tissue of the endocardium.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 733 [Terms] [Paper]