248 



Methods in Plant Histology 



Oogenesis.--The ovules of the Cycads and Ginkgo are very large, 

 and, when mature, thin sections cannot be cut by any method yet 

 discovered. In younger stages it is not difficult to get good sections 



FIG. 86. Ceratozamia mexicana: photomicrograph of a section of a mature sperm 

 showing the large nucleus (very black in the photo) , the cy toplasmic sheath with sections 

 of the ciliated band and cilia; fixed in Flemming's weaker solution and stained in iron- 

 alum haematoxylin. X473. 



of the entire ovule. Slabs should be cut from two sides of the ovule 

 to facilitate fixing and infiltration. During free nuclear stages in the 

 endosperm, and even during earlier stages in the formation of walls, 

 care must be taken that the slabs may not cut into the endosperm, 



