Int. J. Dev. Biol. 58: 889 - 894 (2014)

https://doi.org/10.1387/ijdb.140239db

Vol 58, Issue 10-11-12

Special Issue: Developmental Herpetology

Amniote yolk sacs: diversity in reptiles and a hypothesis on their origin

Published: 2 July 2015

Richard P. Elinson¹, James R. Stewart², Laurie J. Bonneau³ and Daniel G. Blackburn*^,3

¹240 West Neck Road, Huntington, NY, ²Department of Biological Sciences, East Tennessee State University, Johnson City, TN and ³Department of Biology, and Electron Microscopy Center, Trinity College, Hartford, CT, USA

Abstract

Oviparous amniotes produce a large yolky egg that gives rise to a free-living hatchling. Structural characteristics and functional attributes of the egg are best known for birds, which have a large mass of fluid yolk surrounded by an extraembryonic yolk sac. Yolk nutrients are delivered to the embryo via the vascular yolk sac. This developmental pattern and nutrient transport mechanism is thought to be representative of all other lineages of amniotes. Recent discovery of a snake with cellularized yolk organized around a meshwork of blood vessels reveals an additional pattern for yolk mobilization, which may also occur in other squamate reptiles (lizards and snakes). This complex yolk sac raises interesting questions about developmental mechanisms and suggests a possible model for the transition between the egg of anamniotes and that of amniotes.

Keywords

cleavage, yolk, embryo maintenance, yolk sac, oviparity