2IO MORPHOLOGY OF GYMNOSPERMS 



curved, its apex being directed toward the archegonial chamber, 

 and discharges its contents. The sperms have been observed swim- 

 ming in the archegonial chamber, from which one of them passes to 

 the egg and enters its cytoplasm. Ikeno (30) states that the male 

 nucleus slips from its cytoplasmic and cilia-bearing sheath upon 

 entering the cytoplasm of the egg. At the time of fusion the two nuclei 

 are very unequal, the female nucleus being about ten times larger 

 than the male. The mode of fusion is said to be like that described 

 by Ikeno for Cycas revoluta; that is, the sperm nucleus gradually 

 penetrates the egg nucleus before losing its own membrane. It has 

 been determined that fertilization occurs while the ovules are still 

 on the tree (30). 



4. The embryo 



The embryo of Ginkgo has long been regarded as peculiar among 

 gymnosperms, chiefly on account of the absence of a conspicuously 

 elongated suspensor, such as occurs in Cycadales and Coniferales. 

 The only suggested resemblance is the embryo of Bennettitales, in 

 which no suspensor has been seen. Arnoldi (34) recognized a 

 differentiation of the elongating proembryo into three regions: a 

 micropylar haustorial region, a middle suspensor region, and an 

 apical region which gives rise to the embryo proper. This interpre- 

 tation conforms in general to the three regions found in other gymno- 

 sperm embryos. 



Another peculiarity of the embryo of Ginkgo that has been remarked 

 is the complete filling of the egg with proembryonic tissue, but this 

 peculiarity has held only in comparison with the Coniferales, Dioon 

 and perhaps other cycads having the same character. 



In 1904 Lyon (39) published an account of the embryogeny of 

 Ginkgo, and the following description is largely based upon his results 

 as confirmed by our own preparations. The first spindle is very broad 

 and multipolar (30), and a series of simultaneous, free nuclear divi- 

 sions follows, until the nuclei approximate 256 in number (fig. 245). 

 The nuclei do not become parietally placed by the vacuolation of 

 the cytoplasm, but are distributed more or less uniformly throughout 

 .the enlarging egg. At this stage wall-formation occurs, and the egg 

 is filled by a proembryo consisting of undifferentiated tissue (fig. 246). 



