8 Invertebrate Zoology. 



The ova or egg-cells, though often as large as the sperm- 

 spheres, are provided with only a single large nucleus. In 

 this latter may rest a nucleolus. The egg-cells are often 

 amoeboid in form. They are seldom spherical. 



Make drawings of ova and sperm-spheres. 



Segmenting eggs are not infrequently found. Such eggs 

 often have a clear space around them and between their 

 several segments or blastomeres. Two- and four-celled 

 stages are most abundant. Are all the blastomeres provided 

 with nuclei? Can you find an unsegmented ovum that has 

 two nuclei? 



If a cluster of clean sponges be placed in two per cent 

 chromic acid for a day and then carefully washed, stained, 

 imbedded in celloidin and cut transversely, the arrangement 

 of the cell- layers and the structure of the segmenting ova 

 will be more clearly shown. The endoderm will be found 

 to consist of closely appressed, somewhat columnar cells, 

 forming a layer considerably thinner than the outer, less dis- 

 tinct mesoderm. The delicate ectoderm will probably be 

 destroyed. 



The spicules have been dissolved by the chromic acid, 

 but the cells once surrounding them are now seen to be 

 quite irregular in outline and to hold in their meshes the 

 .ova and the sperm-spheres. 



Make a drawing of the transverse section. 



