What allows for the more complex morphology of humans relative to those of simple invertebrates such as nematodes?

The answer is; endoskeleton

An endoskeleton allows for smooth and steady growth unlike in an invertebrate with an exoskeleton that needs to shed their skeleton during irregular growth spurts. An endoskeleton is significant for leveraging (like an anchor) skeletal muscles hence enabling the animals to grow big and is able to support their own weight.

prophase: we can observe the chromosomes to coil up and condense. they become visible. nuclear membrane and envelope disintigrate.s

metaphase: chromosomes align at the center, the metaphase plate. spindle fibers attach to centromeres of the sister chromatids. they meet at kinetochores.

anaphase: sister chromatids are pulled towards each opposing centromere, set on different poles.

telophase: chromosomes are now clustered up only on either sides of the cell. nuclear membranes reform, and a new cell wall forms along the cell plate. this will lead to cytokinesis, the process that divides the cell membrane and forms the cell wall.

the unequal cytoplasmic division that  results  in  one egg  and three polar bodies ensures that a fertilized  egg has  ample nutrients  forits journey  to  the uterus.