426 MORPHOLOGY OF GYMNOSPERMS 



chromatin contributed by the two parents. Whether a blending 

 occurs as the two groups of chromosomes pass from the telophase 

 of the first mitosis into the resting reticulums of the daughter nuclei 

 has not been determined, but it is possible that the male and female 

 chromatin may remain distinct throughout the life history. 



In all the genera investigated, more or less cytoplasm enters the 

 egg with the male nucleus. In the cycads the entire sperm enters 

 the egg, and the cilia may continue to move in this environment, but 

 the nucleus soon slips out from its cytoplasmic sheath and advances 

 toward the egg nucleus. In other forms, as Torreya, Juniperus, 

 and Taxodium, the cytoplasm of the male cell invests the fusion 

 nucleus and takes part in the formation of the proembryo. In most 

 genera, however, no such cytoplasm is visible, and the proembryo 

 is formed from a rather small portion of the basal region of the egg, 

 quite remote from any cytoplasm that may have entered the egg with 

 the male nucleus. 



THE EMBRYO 



The absence of embryos from the seeds of paleozoic gymnosperms 

 has been taken to indicate that some great change connected with 

 embryo-formation was introduced by the mesozoic gymnosperms. 

 It would be of extreme interest to know the ancient condition, but 

 we know only what it has become. In fact, the evidence is by no 

 means clear that the embryogeny of paleozoic gymnosperms differed 

 in any general way from that of later gymnosperms. Attention has 

 been called (p. 47) to the fact that the paleozoic seeds sectioned had 

 dropped from their plants just as abortive seeds do, and that no 

 sections of paleozoic seeds in connection with their plants have been 

 obtained. Among Bennettitales attached seeds are the only ones 

 sectioned, and they contain embryos. Until sections of attached 

 paleozoic seeds persistently show the absence of embryos, it is not 

 profitable to imagine what may have been the changes in embryo- 

 development in passing from the Paleozoic into the Mesozoic. What- 

 ever may have been the changes, if any, the proembryo has become 

 the structure showing steady and progressive changes. 



The first stage in the development of the proembryo is free nuclear 

 division, followed by wall-formation; and in the most primitive 



