Introduction

Almost all the experimental analyses of the embryology of birds have been carried out on the domestic fowl (Gallus gallus), though sufficient work has been published on other domesticated species to indicate that there is little variation in the patterns of development, at least among the carinates. The situation is sadly different for reptiles, where there has been little experimental work, and even morphological studies are still needed for some of the most critical stages of certain groups. Some patterns emerge, but owing to the wide variation among different groups of reptiles we must maintain great caution in making generalisations. This chapter is concerned principally with the patterns of development which are evident at the earliest stages, i.e. until the beginning of organogenesis. It is during this period that the basic events of embryogenesis take place and the patterns of development that emerge are fundamental for the establishment of the entire embryonic body. Important differences which occur at later stages and play an essential role in the divergence of body form are well covered in a series of reviews in The Biology of the Reptilia (Ewert, 1985; Ferguson, 1985; Hubert, 1985a,b; Moffat, 1985) and will not be discussed here.

The ovum and fertilisation

Birds and reptiles have many shared characteristics, not least of which is the possession of the cleidoic tgg. The large yolk is formed from lipids and proteins which are transmitted from the maternal circulation through the follicle cells into the ovum (White, Chapter 1).