During development two strongly interrelated processes can be discerned in the central nervous system (eNS), namely morphogenesis and histogenesis. Most neuroembryological studies deal with histogenetic features virtually with- out any morphological elucidation. It must be stressed, however, that histogen- etic investigations should be based upon a thorough knowledge of morphogene- sis. This holds especially for the forebrain, which during development is sub- jected to drastic transformations, particularly when only two-dimensional sec- tions are used. Therefore the present study on morphogenesis forms the first part of a research project on the ontogenesis of the brain in the rhesus monkey. The second part (Gribnau and Geijsberts 1984) will deal with the early histogene- sis of the forebrain. The first recognizable precursor of the eNS in vertebrates is the neural plate, which, after the formation of the germ layers, is induced in the ectoderm. The lateral margins of the neural plate start to rise, forming a neural groove. Eventu- ally, they meet dorsally in the midline and fuse, resulting in the formation of the neural tube. The ultimate sites of closure at either end of the neural tube are called the anterior and posterior neuropores. Before the closure of the anterior neuropore, which precedes that of the posterior neuropore, the anlage of the eNS can be divided into a narrow elongated caudal part, the future spinal cord, and a wider rostral part, the precursor of the brain.

In the past decennia nonhuman primates have been increasingly used for research purposes in various scientific fields. Much interest has been focused on this group of animals in general and on the rhesus monkey in particular because of its close phylo genetic relationship with man. In some fields of research, however, such as embryology and microscopic anatomy, much less attention has been paid to nonhuman primates, probably because of the expense involved in the collection of the extensive material needed. On the other hand, teratological and experimental embryologic studies must be based upon a thorough knowledge of the normal ontogenesis since only in that way can a reliable distinction be made between normal and abnormal or induced develop ment. Each ontogenetic study essentially consists of a comparison of different deve lopmental stages. In most reports dealing with the development of individual organs or their subunits the material used is classified according to the estimated age or the length of the embryos. These criteria, however, are not valid, since considerable varia tion in developmental stage occurs between animals of the same age even between littermates and between animals of the same length. Therefore a method is needed for assigning embryos to successive developmental stages that are defmed on the basis of extemal and internal characteristics. This type of classification was elaborated by Stree ter (1942, 1945, 1948,1951), who arranged human embryos into developmental hori zons numbered XI through XXIII."