In doing this, the queen is doubtless controlled by 

 temperature, food, the conditions in the hive, the size of 

 the cells provided for the eggs, etc. When she forces out 

 sperms on the passing eggs and they are fertilized, they 

 develop into the workers (females). If the eggs are not 

 fertilized, they develop into the males (drones). While 

 the workers are females, they are not perfectly developed 

 females as the queen is. Apparently any fertilized egg 

 may develop a queen if the larva is specially nourished to 

 that end. Observers claim that the workers may lay 

 unfertilized eggs that may develop. There are many cases 

 of parthenogenesis found among the kindred of the bees, 

 the other social insects, such as bumble bees, wasps, ants, 

 etc. Some of these are as interesting as the case of the 

 honey bee, but they are not so well known to the general 

 student. 



4. Hydatina, the Rotifer. This little "wheel animalcule," 

 of about the grade of development shown by worms, is 

 found in stagnant water. Its life history has been very 

 well studied. There are three kinds of eggs produced, 

 "summer eggs'' of two sizes, and "winter eggs." The 

 summer eggs are thin-shelled and are not fertilized. The 

 larger ones develop at once and produce females. These 

 females may in rapid succession produce several 

 generations of parthenogenetic females. Later in the 

 season, these females lay the thick-shelled winter eggs, 

 which must be fertilized in order to develop. About this 

 time the males appear, hatched, seemingly, from the 

 smaller summer eggs. After fertilization, the winter eggs 

 lie dormant through the winter and develop, with the 

 return of warmth, into the females with which we began. 



It has been found that external conditions, such as 

 temperature, state of the water, food, etc., have a part in 

 determining the kinds of eggs produced and the length 

 of the parthenogenetic generations. 



5. The Aphids, or Plant Lice. A condition similar in 

 many respects to the last is found in the aphids, a group 

 of small sucking insects that are parasitic on the tender 

 twigs, roots, and leaves of many plants. In the autumn 

 there are females and males, and the eggs are fertilized. 

 These eggs are attached to the twigs of the host plants, 

 in crevices where they may be partly protected. In the 

 spring they hatch out into females, which in their turn lay 

 eggs that hatch without fertilization. All the offspring at 

 this time are females with this power of laying 

 parthenogenetic eggs. This may continue for ten or 

 twelve generations, giving rise to an unthinkable number 

 of offspring which may be a great tax on the host plant. 

 Finally, in the autumn, a generation is formed that 

 includes both males and females. These mate and the 

 fertilized eggs go over the winter, as described at the 

 beginning, until the folowing spring. In some aphid 

 mothers, the parthenogenetic eggs develop before they 

 leave the body and the young are thus brought forth alive 

 (vivipary). 



6. Parthenogenesis and Non-Sexual Reproduction. At 

 first thought it might seem that parthenogenesis is just 

 one of the non-sexual methods of reproduction, like 

 budding or spore formation. A spore is a reproductive 

 cell that can develop without fertilization. Then why is 

 not parthenogenesis a form of spore reproduction? Two 

 or three facts, however, make us realize that 

 parthenogenesis is more related to sexual reproduction. 

 In the first place, the parthenogenetic cells are produced 

 in ovaries, where eggs arise; and in the second place, in 

 such forms as the bees it seems that any egg may become 

 either a fertilized egg or a parthenogenetic one. It would 

 seem, therefore, that parthenogenesis is a case where the 

 process has gone backward; where a cell that might be 



expected to unite has given up union and has come to 

 develop without it. 



CHAPTER SIXTEEN. 

 THE FAMILY OF THE FROGS 



1. The Family Tree. A great many of our examples in 

 the' earlier part of this book have been taken from the 

 plants and the lower animals, the invertebrates. The 

 frogs, along with the fishes, birds, and men, are vertebrates. 

 The class of vertebrates to which frogs belong is not a 

 very large or important one, but it is one of the very most 

 interesting because of its method of reproduction and 

 development. In the family are frogs and toads, tail-less 

 forms, including many kinds of tree toads; and a number 

 of slimy animals with tails, somewhat like lizards, that 

 live in water or moist places. These latter are newts, 

 salamanders, sirens, "mud-dogs," and the like. 



The frogs and toads breathe in the air, and may live on 

 land, but they (the frogs, particularly) spend much of 

 their time in or near the water. Most of the family would 

 die if long out of water. 



2. Reproductive Habits of the Frog. The frogs have 

 two sexes. The mother lays large masses of eggs in the 

 water. The egg proper, which is about the size of medium 

 shot, is dark in color and is surrounded by a layer of clear 

 transparent gelatin which helps to hold the eggs together 

 in globular masses or in strings, depending on the species. 

 Frogs do not lay eggs in nests, or specially prepared 

 places; but the frogs and even the land toads go regularly 

 in early spring to the shallow margins of pools and 

 streams. The eggs are frequently attached by the gelatin 

 to sticks or leaves of grasses or to other stationary objects 

 in the water. 



3. Mating and Fertilization. While the mother is laying 

 the eggs in the water, the male frog pours out great 

 numbers of sperm cells over them, also in the water. 

 These sperms can swim, and under the attraction of the 

 eggs one sperm cell unites with each of the eggs. The 

 gelatin is not a living part of the egg, and therefore the 

 sperm must pass through this until it reaches the dark 

 cell within. The head of the sperm containing the nucleus 

 passes into the egg proper and unites completely with the 

 nucleus of the egg. This forms a fertilized egg or embryo 

 with all the powers of developing into a mature frog. 



4. The Early Development. If we were watching one of 

 these eggs with a microscope, we should see, about two 

 hours after fertilization, a little groove passing round the 

 egg and marking it off into two hemispheres. This shows 

 that the one-celled embryo has now become two-celled. 

 A little later it divides again, at right angles to the first 

 division, into four cells. These four cells are equal. Soon 

 other walls appear at right angles to both of those 

 previously formed. These new walls divide each of the 

 four cells into two, one of which (the upper) is consider- 

 ably smaller than the other. Divisions of all the cells 

 continue to take place rapidly. By the end of twenty-four 

 hours there are too many cells to count. 



During this time the embryo, while changing its shape a 

 little, is not larger than it was at the start, yet there are 

 hundreds of cells where there was one at first. It follows 

 then that the cells in dividing have become much smaller. 



