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The early planarian embryo presents a complete ciliated epidermis and a pharynx and feeds on maternal yolk cells. In this paper, we report on all the elements involved in the formation of such an autonomous embryo, which we name cryptic larva. First, we provide a description of the spherical and fusiform yolk cells and their relationship with the blastomeres, from the laying of the egg capsule up to their final fate in mid embryonic stages. Then, we describe the early cleavage and the subsequent development of the tissues of the cryptic larva, namely, the primary epidermis, the embryonic pharynx, and a new cell type, the star cells. Finally, we discuss the possibility that the cryptic larva either constitutes a vestigial larva or, more likely, is the evolutionary result of the competition between multiple embryos for the limited and shared maternal resources in the egg capsule. [ABSTRACT FROM AUTHOR]

The monitoring of gene expression is fundamental for understanding developmental biology. Here we report a successful experimental protocol for in situ hybridization in both whole-mount and sectioned planarian embryos. Conventional in situ hybridization techniques in developmental biology are used on whole-mount preparations. However, given that the inherent lack of external morphological markers in planarian embryos hinders the proper interpretation of gene expression data in whole-mount preparations, here we used sectioned material. We discuss the advantages of sectioned versus whole-mount preparations, namely, better probe penetration, improved tissue preservation, and the possibility to interpret gene expression in relation to internal morphological markers such as the epidermis, the embryonic and definitive pharynges, and the gastrodermis. Optimal fixatives and embedding methods for sectioning are also discussed. [ABSTRACT FROM AUTHOR]